Rename a CMake variable in compile_flags.cmake to avoid triggering
CMake 3.1 warning about CMP0054 about interpreting if() arguments
as keywords or variables.
No changes in behavior.
Bug#17959689: MAKE GCC AND CLANG GIVE CONSISTENT COMPILATION WARNINGS
Bug#18313717: ENABLE -WERROR IN MAINTANER MODE WHEN COMPILING WITH CLANG
Bug#18510941: REMOVE CMAKE WORKAROUNDS FOR OLDER VERSIONS OF OS X/XCODE
Backport from mysql-5.6 to mysql-5.5