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This compiler id was added by commit 90c6156a (XLClang: Add a new
compiler ID for the clang-based XL compiler, 2019-02-05,
cpp-modules-20190312.1~71^2~7).  Add documentation accidentally left out
of that commit.

Issue: #18835

Release versions do not have the development topic section of
the CMake Release Notes index page.

Add section headers similar to the 3.14 release notes and move each
individual bullet into an appropriate section.  Revise a few bullets.

Run the `Utilities/Release/consolidate-relnotes.bash` script to move
notes from `Help/release/dev/*` into `Help/release/3.15.rst`.

Copy wording from the release notes.

The `make_directory` command can make multiple directories in a single
invocation.  Make `remove_directory` mirror that behavior.

This allows for expressions such as:

 $<COMPILE_LANG_AND_ID, CXX, GNU, Clang>

This allows for expressions like:
 $<$<CXX_COMPILER_ID:Clang,GNU>:-DMY_PRIVATE_DEFINE>

Fixes: #19321

Co-Authored-By: Steve Mokris <smokris@softpixel.com>

Entries of the `CPATH` environment variable are implicitly searched as
include directories by some C/C++ compilers.  Since commit 5990ecb7
(Compute implicit include directories from compiler output, 2018-12-07,
v3.14.0-rc1~108^2) these entries are detected by CMake and included in
the `CMAKE_{C,CXX}_IMPLICIT_INCLUDE_DIRECTORIES` variables.

However, we should not exclude them from explicit specification via `-I`
or particularly `-isystem` because they are meant as user-specified
include directories that can be re-ordered without breaking compiler
builtin headers.  In particular, we need explicit requests via
`include_directories` with the `SYSTEM` option to result in `-isystem`
so that third-party headers do not produce warnings.

Co-Author: Ben Boeckel <ben.boeckel@kitware.com>
Fixes: #19291

Our practice of closing MRs temporarily while discussion
takes place in a separate issue isn't always well understood
by MR authors. Expiring a MR seems to be better understood,
but making it clear that it is also a temporary state is helpful.

These variables apply to the `ctest_build` command, not `ctest_test`

Relates: #19217

Read the version from `Source/CMakeVersion.cmake` instead of repeating it.

Documentation for environment variables that control the default
generator selection:

* CMAKE_GENERATOR
* CMAKE_GENERATOR_INSTANCE
* CMAKE_GENERATOR_PLATFORM
* CMAKE_GENERATOR_TOOLSET

Fixes #18657

Add a `CMAKE_FIND_PACKAGE_PREFER_CONFIG` variable to tell
`find_package` calls to look for a package configuration file
first even if a find module is available.

Fixes: #16805, #19236

Previously code like

    file(REMOVE_RECURSE "${accidentally_missing_variable}")

treated the empty string as a relative path with respect to the
current directory and removed its contents.  Change this behavior
to ignore the empty string with a warning instead.

Normally such behavior changes are done with a policy, but in this case
such code is likely a real bug in project code that can delete data.

Fixes: #19274

* Add find_library calls for debug and release
* Set imported configs for corresponding libraries in imported target
* Set debug/optimized in PostgreSQL_LIBRARIES
Signed-off-by: Andrei Lebedev <lebdron@gmail.com>

Fixes: #19261

Fixes: #19264