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This change was originally made by !1620 (merged) but it was reverted by !1931 (merged) because the behavior change could affect checks in existing projects in an incompatible way.
Fixes: #9514 (closed)