CheckIncludeFiles: Honor CMAKE_REQUIRED_LIBRARIES

Other check modules honor this variable, so include file checks should
too.  Add policy `CMP0075` to enable the behavior in a compatible way.

This change was originally made by commit v3.11.0-rc1~108^2
(CheckIncludeFiles: Honor CMAKE_REQUIRED_LIBRARIES, 2017-12-24) but it
was reverted by commit v3.11.1~9^2 (Revert "CheckIncludeFiles: Honor
CMAKE_REQUIRED_LIBRARIES", 2018-04-04) because the behavior change could
affect checks in existing projects in an incompatible way.

Fixes: #9514

1 files changed, 26 insertions, 0 deletions

diff --git a/Help/policy/CMP0075.rst b/Help/policy/CMP0075.rst
new file mode 100644
index 0000000..aa5c3f7
--- /dev/null
+++ b/Help/policy/CMP0075.rst
@@ -0,0 +1,26 @@
+CMP0075
+-------
+
+Include file check macros honor ``CMAKE_REQUIRED_LIBRARIES``.
+
+In CMake 3.12 and above, the
+
+* ``check_include_file`` macro in the :module:`CheckIncludeFile` module, the
+* ``check_include_file_cxx`` macro in the
+  :module:`CheckIncludeFileCXX` module, and the
+* ``check_include_files`` macro in the :module:`CheckIncludeFiles` module
+
+now prefer to link the check executable to the libraries listed in the
+``CMAKE_REQUIRED_LIBRARIES`` variable.  This policy provides compatibility
+with projects that have not been updated to expect this behavior.
+
+The ``OLD`` behavior for this policy is to ignore ``CMAKE_REQUIRED_LIBRARIES``
+in the include file check macros.  The ``NEW`` behavior of this policy is to
+honor ``CMAKE_REQUIRED_LIBRARIES`` in the include file check macros.
+
+This policy was introduced in CMake version 3.12.  CMake version
+|release| warns when the policy is not set and uses ``OLD`` behavior.
+Use the :command:`cmake_policy` command to set it to ``OLD`` or ``NEW``
+explicitly.
+
+.. include:: DEPRECATED.txt