All generators now support detection of the full path to the compiler, so
require it to be so.  This will allow CMake<LANG>Information.cmake and
other logic to assume the full path to the compiler tool is available.

The Makefile generators already rejected CMAKE_<LANG>_COMPILER values
that did not name an existing compiler.  Extend this error message to
all generators, make it occur as early as possible, and improve the
message with advice about how to set the compiler.  If the full path to
the compiler is not known, finish enabling languages with a fatal error
so configuration does not continue.

For now, allow the RC language compiler to not be a full path.  Later we
will need to detect the full path to "rc" under the VS IDE.

Add a RunCMake.CompilerNotFound test to cover failure cases.
Fix the RunCMake.CompilerChange test EmptyCompiler case to work
when configuration does not continue past enable_language.

Only valid target names or generator expressions may appear in
the target field of a LINK_ONLY expression.

Other content like link flags should still be added to that property
(wrapped in config-specific generator expressions), but not wrapped
in LINK_ONLY. Otherwise undue warnings would be issued for the
policy CMP0022.

The LINK_ONLY expression only has an effect for actual target
names anyway, so there is no logical deficit.

Set the minimum required version of CMake high enough to avoid the
warning for CMAKE_LEGACY_CYGWIN_WIN32.  The warning appears on stderr
and breaks the expected output matching.

It was a subtest of the RunCMake.ObjectLibrary test. However, we need
to test a build with ExternalProject after running CMake, which RunCMake tests
do not do.

Teach the CMake language lexer to treat the \-LF pair terminating a
line ending in an odd number of backslashes inside a quoted argument
as a continuation.  Drop the pair from the returned quoted argument
token text.  This will allow long lines inside quoted argument
strings to be divided across multiple lines in the source file.
It will also allow quoted argument text to start on the line after
the opening quote.  For example, the code:

 set(x "\
 ...")

sets variable "x" to the value "..." with no opening newline.

Previously an odd number of backslashes at the end of a line inside
a quoted argument would put a \-LF pair (or a \-CR pair) literally
in the argument.  Then the command-argument evaluator would complain
that the \-escape sequence is invalid.  Therefore this syntax is
available to use without changing behavior of valid existing code.

Teach the RunCMake.Syntax test to cover cases of quoted arguments
with lines ending in \, \\, and \\\.  Odd counts are continuations.

Drop all behavior activated by setting CMAKE_BACKWARDS_COMPATIBILITY to
a value lower than 2.4, and generate an error when projects or the user
attempt to do so.  In the error suggest using a CMake 2.8.x release.

Teach cmake_minimum_required to warn about projects that do not require
at least CMake 2.4.  They are not supported by CMake >= 3.0.

Replace the documentation of CMAKE_BACKWARDS_COMPATIBILITY with a
reference to policy CMP0001.

Other warnings for the same policy already have similar output since
commit 81d2793e (Add differing target property content to policy CMP0022
warning, 2013-09-11).

Add cmCommand::Disallowed helper to check the setting of a policy that
disallows the command.  Add a RunCMake.DisallowedCommands test
placeholder.  Add a Help/policy/DISALLOWED_COMMAND.txt file for
inclusion by each policy document to avoid duplication of the common
text.

Add a string(CONCAT) command to simply concatenate input arguments
together.  This will be useful for combining strings from different
quoting syntaxes.  Add a RunCMake.string test covering these cases.

Introduce a policy to control the behavior.

The AliasTargets unit test already tests that using a
double-semicolon in the name is not an error. Change the ExportImport
test to use a namespace with a double-semicolon too.

Set the minimum required version of CMake high enough to avoid the
warning for CMAKE_LEGACY_CYGWIN_WIN32.  The warning appears on stderr
and breaks the expected output matching.

Teach the CMake language parser to recognize Lua-style "long bracket"
arguments.  These start with two '[' separated by zero or more '='
characters e.g. "[[" or "[=[" or "[==[".  They end with two ']'
separated by the same number of '=' as the opening bracket.  There is no
nesting of brackets of the same level (number of '=').  No escapes,
variable expansion, or other processing is performed on the content
between such brackets so they always represent exactly one argument.

Also teach CMake to parse and ignore "long comment" syntax.  A long
comment starts with "#" immediately followed by an opening long bracket.
It ends at the matching close long bracket.

Teach the RunCMake.Syntax test to cover long bracket and long comment
cases.

Read input files in binary mode instead of text mode and convert CRLF
newlines to LF newlines explicitly in our own buffer.  This is necessary
to read CMake source files with CRLF newlines on platforms whose C
runtime libraries do not transform newlines in text mode.  For example,
a Cygwin or Linux binary may not transform CRLF -> LF in files read from
a Windows filesystem.  Perform the conversion ourselves to ensure that
multi-line string literals in CMake source files have LF newlines
everywhere.

Teach the lexer to read a UTF-8, UTF-16 BE/LE, or UTF-32 BE/LE
Byte-Order-Mark from the start of a file if any is present.  Report an
error on files using UTF-16 or UTF-32 and accept a UTF-8 or missing BOM.

Teach the lexer to treat a single letter as an identifier instead of an
unquoted argument.  Outside of a command invocation, the parser treats
an identifier as a command name and an unquoted argument as an error.
Inside of a command invocation, the parser treats an identifier as an
unquoted argument.  Therefore this change to the lexer will make what
was previously an error case work with no other behavioral change.

The final location and name of a build-target is not determined
until generate-time. However, reading the LOCATION property from
a target is currently allowed at configure time. Apart from creating
possibly-erroneous results, this has an impact on the implementation
of cmake itself, and prevents some major cleanups from being made.

Disallow reading LOCATION from build-targets with a policy. Port some
existing uses of it in CMake itself to use the TARGET_FILE generator
expression.

Make the API for adding targets string based so that it can easily
use cmGeneratorTarget.

Teach the cmIncludeCommand to generate the exported file at
configure-time instead if it is to be include()d.

The RunCMake.ExportWithoutLanguage test now needs a dummy header.h
file as expected error from export() is now reported after the
missing file error.

Storing it in the makefile means that the policy does not trigger
when include and export are in differing directories.

By default, the message is not issued. If CMAKE_ERROR_DEPRECATED
is on, the message is fatal. If CMAKE_WARN_DEPRECATED is on, the
message is a warning.

Previously if headers required to check if a struct has a member can be
compiled with C++ compiler only, the check would fail because the C
compiler fails.  As a consequence, the result variable would be set to
false, even if the struct has that particular member.

Teach CHECK_STRUCT_HAS_MEMBER to accept a new optional argument LANGUAGE
that allows one to explicitly set the compiler to use.  The new
signature is therefore:

  CHECK_STRUCT_HAS_MEMBER (<struct> <member> <header> <variable>
                           [LANGUAGE <language>])

CMake is aware of the policy's NEW behavior and the AppleClang compiler
id.  Set the policy to NEW explicitly to avoid the warning and get the
NEW behavior.

Also teach the RunCMake test infrastructure to build tests with
-DCMAKE_POLICY_DEFAULT_CMP0025=NEW to avoid the policy warning
in test output that must match specific regular expressions.

Currently, export() is executed at configure-time.

One problem with this is that certain exported properties like
the link interface may not be complete at the point the export() is
encountered leading to an incorrect or incomplete exported
representation. Additionally, the generated IMPORTED_LOCATION
property may even be incorrect if commands following the export()
have an effect on it.

Another problem is that it requires the C++ implementation of cmake
to be capable of computing the exported information at configure time.
This is a limitation on the cleanup and maintenance of the code. At
some point in the future, this limitation will be dropped and more
implementation will be moved from cmTarget to cmGeneratorTarget.

This target type only contains INTERFACE_* properties, so it can be
used as a structural node. The target-specific commands enforce
that they may only be used with the INTERFACE keyword when used
with INTERFACE_LIBRARY targets. The old-style target properties
matching LINK_INTERFACE_LIBRARIES_<CONFIG> are always ignored for
this target type.

The name of the INTERFACE_LIBRARY must match a validity generator
expression. The validity is similar to that of an ALIAS target,
but with the additional restriction that it may not contain
double colons. Double colons will carry the meaning of IMPORTED
or ALIAS targets in CMake 2.8.13.

An ALIAS target may be created for an INTERFACE library.

At this point it can not be exported and does not appear in the
buildsystem and project files are not created for them. That may
be added as a feature in a later commit.

The generators need some changes to handle the INTERFACE_LIBRARY
targets returned by cmComputeLinkInterface::GetItems. The Ninja
generator does not use that API, so it doesn't require changes
related to that.

When RunCMake tests run during dynamic analysis, valgrind may add lines
of the form "==[0-9]+==..." to the output.  Remove such lines from the
actual output before matching it against the expected output.

List the contents of the INTERFACE_LINK_LIBRARIES and the old-style
property.

Teach FailCopyFileABI-check.cmake to convert CRLF to LF in the regular
expression read literally from its own source.