The OLD behaviors of all policies are deprecated, but only by
documentation. Add an explicit deprecation diagnostic for policies
introduced in CMake 3.12 and below to encourage projects to port
away from setting policies to OLD.
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This change was originally made by commit v3.11.0-rc1~108^2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
Brad King