try_compile: Add policy CMP0067 to honor language standards

Projects use `try_compile` to check if they will be able to compile some
particular source code.  When a language standard variable like
`CMAKE_CXX_STANDARD` is set, then the project intends to compile source
code using a compiler mode for that standard.  Therefore it makes sense
for `try_compile` to use that standard in the test project too.

Unfortunately this was not done when support for the
`CMAKE_CXX_STANDARD` variable was first implemented.  Add a policy to
introduce the improved behavior in a compatible way.

Closes: #16456

1 files changed, 14 insertions, 0 deletions

diff --git a/Help/command/try_compile.rst b/Help/command/try_compile.rst
index 3f16b63..cde3776 100644
--- a/Help/command/try_compile.rst
+++ b/Help/command/try_compile.rst
@@ -135,3 +135,17 @@ the type of target used for the source file signature.
 Set the :variable:`CMAKE_TRY_COMPILE_PLATFORM_VARIABLES` variable to specify
 variables that must be propagated into the test project.  This variable is
 meant for use only in toolchain files.
+
+If :policy:`CMP0067` is set to ``NEW``, or any of the ``<LANG>_STANDARD``,
+``<LANG>_STANDARD_REQUIRED``, or ``<LANG>_EXTENSIONS`` options are used,
+then the language standard variables are honored:
+
+* :variable:`CMAKE_C_STANDARD`
+* :variable:`CMAKE_C_STANDARD_REQUIRED`
+* :variable:`CMAKE_C_EXTENSIONS`
+* :variable:`CMAKE_CXX_STANDARD`
+* :variable:`CMAKE_CXX_STANDARD_REQUIRED`
+* :variable:`CMAKE_CXX_EXTENSIONS`
+
+Their values are used to set the corresponding target properties in
+the generated project (unless overridden by an explicit option).