In the `try_compile` source file signature we propagate the caller's
value of `CMAKE_<LANG>_FLAGS` into the test project.  Extend this to
propagate `CMAKE_<LANG>_FLAGS_<CONFIG>` too instead of always using the
default value in the test project.  This will be useful, for example, to
allow the MSVC runtime library to be changed (e.g. `-MDd` => `-MTd`).
However, some projects may currently depend on this not being done,
so we need to activate the behavior using a policy.

This change was originally made by commit v3.6.0-rc1~160^2 (try_compile:
Honor CMAKE_<LANG>_FLAGS_<CONFIG> changes, 2016-04-11) but without the
policy and so had to be reverted during the 3.6 release candidate cycle.

Fixes #16174.