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This change was originally made by commit v3.11.0-rc1~108^2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
Previously if headers required to check if a struct has a member can be
compiled with C++ compiler only, the check would fail because the C
compiler fails. As a consequence, the result variable would be set to
false, even if the struct has that particular member.
Teach CHECK_STRUCT_HAS_MEMBER to accept a new optional argument LANGUAGE
that allows one to explicitly set the compiler to use. The new
signature is therefore:
CHECK_STRUCT_HAS_MEMBER (<struct> <member> <header> <variable>
[LANGUAGE <language>])
