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This command is intended for modules that issue policy warnings so
they can get the warning string from CMake in a uniform manner,
rather than duplicating the string. Several modules been updated
to include an example of the usage of this new command.

We now require C++11 support including `override`.  Drop use of
the old compatibility macro.  Convert references as follows:

    git grep -l CM_OVERRIDE -- '*.h' '*.hxx' '*.cxx' |
      xargs sed -i 's/CM_OVERRIDE/override/g'

Also remove `#include "cmConfigure.h"` from most source files.

Automate with:

git grep -l '#include <cm_' -- Source \
  | xargs sed -i 's/#include <\(cm_.*\)>/#include "\1"/g'

git grep -l '#include <cmsys/' -- Source \
  | xargs sed -i 's/#include <\(cmsys\/.*\)>/#include "\1"/g'

git grep -l '#include <cm[A-Z]' -- Source \
  | xargs sed -i 's/#include <\(cm[A-Z].*\)>/#include "\1"/g'

Make sure that `#include <cmConfigure.h>` is followed by an empty line
in header files.  This is necessary to make sure that changing <> to ""
does not affect the include ordering of clang-format.

Automate with:

git grep -l '#include <cmConfigure.h>' | grep -v '.cxx$' \
| xargs sed -i '/#include <cmConfigure.h>/ { N; N; s/\n\{1,2\}/\n\n/ }'

Per-source copyright/license notice headers that spell out copyright holder
names and years are hard to maintain and often out-of-date or plain wrong.
Precise contributor information is already maintained automatically by the
version control tool.  Ultimately it is the receiver of a file who is
responsible for determining its licensing status, and per-source notices are
merely a convenience.  Therefore it is simpler and more accurate for
each source to have a generic notice of the license name and references to
more detailed information on copyright holders and full license terms.

Our `Copyright.txt` file now contains a list of Contributors whose names
appeared source-level copyright notices.  It also references version control
history for more precise information.  Therefore we no longer need to spell
out the list of Contributors in each source file notice.

Replace CMake per-source copyright/license notice headers with a short
description of the license and links to `Copyright.txt` and online information
available from "https://cmake.org/licensing".  The online URL also handles
cases of modules being copied out of our source into other projects, so we
can drop our notices about replacing links with full license text.

Run the `Utilities/Scripts/filter-notices.bash` script to perform the majority
of the replacements mechanically.  Manually fix up shebang lines and trailing
newlines in a few files.  Manually update the notices in a few files that the
script does not handle.

Run the `Utilities/Scripts/clang-format.bash` script to update
all our C++ code to a new style defined by `.clang-format`.
Use `clang-format` version 3.8.

* If you reached this commit for a line in `git blame`, re-run the blame
  operation starting at the parent of this commit to see older history
  for the content.

* See the parent commit for instructions to rebase a change across this
  style transition commit.

Drop all GetTerseDocumentation and GetFullDocumentation methods from
commands.  The command documentation is now in Help/command/*.rst files.

Our Git commit hooks disallow modification or addition of lines with
trailing whitespace.  Wipe out all remnants of trailing whitespace
everywhere except third-party code.

Run the following shell code:

git ls-files -z -- \
 bootstrap doxygen.config '*.readme' \
 '*.c' '*.cmake' '*.cpp' '*.cxx' \
 '*.el' '*.f' '*.f90' '*.h' '*.in' '*.in.l' '*.java' \
 '*.mm' '*.pike' '*.py' '*.txt' '*.vim' |
egrep -z -v '^(Utilities/cm|Source/(kwsys|CursesDialog/form)/)' |
egrep -z -v '^(Modules/CPack\..*\.in)' |
xargs -0 sed -i 's/ \+$//'

Use const_cast for the special case in cmFindBase where
GetFullDocumentation calls GenerateDocumentation.

Check CMAKE_POLICY_DEFAULT_CMP<NNNN> for a default when policy CMP<NNNN>
would otherwise be left unset.  This allows users to set policies on the
command line when the project does not set them.  One may do this to
quiet warnings or test whether a project will build with new behavior
without modifying code.  There may also be cases when users want to
build an existing project release using new behavior for policies
unknown to the project at the time of the release.

Add the [.tweak] version component throughout the policy implementation.
Document all components for the cmake_policy(VERSION) command.  Record
the tweak level in which each policy was introduced (0 for all current
policies).  In generated documentation we report the tweak level only if
it is not zero.  This preserves existing documentation.

This converts the CMake license to a pure 3-clause OSI-approved BSD
License.  We drop the previous license clause requiring modified
versions to be plainly marked.  We also update the CMake copyright to
cover the full development time range.

The previous documentation could be interpreted as setting policies
newer than the given version to OLD instead of unset.  This clarifies
it.

Isolation of policy changes inside scripts is important for protecting
the including context.  This teaches include() and find_package() to
imply a cmake_policy(PUSH) and cmake_policy(POP) around the scripts they
load, with a NO_POLICY_SCOPE option to disable the behavior.  This also
creates CMake Policy CMP0011 to provide compatibility.  See issue #8192.

This teaches functions and macros to use policies recorded at creation
time when they are invoked.  It restores the policies as a weak policy
stack entry so that any policies set by a function escape to its caller
as before.

This re-organizes the discussion of the policy stack in documentation of
the cmake_policy() command.  The new organization clearer and easier to
extend with new information.

It is likely that projects or CMake modules in the future will need to
check the value of a policy setting.  For example, if we add a policy
that affects the results of FindXYZ.cmake modules, the module code will
need to be able to check the policy.

  - Add a paragraph introducing the policy mechanism
  - Explicitly introduce the CMP<NNNN>, OLD, and NEW notation
  - Note that setting policies by CMake version is preferred
  - Fix SET signature to use CMP<NNNN> notation
  - Add more details about the policy stack

  - Remove CMP_0001 (no slash in target name) and restore
    old CMAKE_BACKWARDS_COMPATIBILITY check for it
  - Replace all checks of CMAKE_BACKWARDS_COMPATIBILITY
    with cmLocalGenerator::NeedBackwardsCompatibility calls
  - Create new CMP_0001 to determine whether or not
    CMAKE_BACKWARDS_COMPATIBILITY is used.
    (old = use, new = ignore)
  - Show CMAKE_BACKWARDS_COMPATIBILITY in cache only when
    CMP_0001 is set to OLD or WARN
  - Update documentation of cmake_policy and cmake_minimum_required
    to indicate their relationship and the 2.4 version boundary
  - When no cmake policy version is set in top level makefile
    implicitly call cmake_policy(VERSION 2.4) which restores
    CMAKE_BACKWARDS_COMPATIBILITY and other 2.4 compatibility
  - Fix tests MakeClean and Preprocess to call
    cmake_policy(VERSION 2.6) because they depend on new policies

  - Replace NEW and OLD modes with a SET mode for clarity
  - Enforce VERSION argument validity (major.minor[.patch])