When determining gcc/clang flags, save resulting C flags as MY_C_FLAGS,
C++ flags as MY_CXX_FLAGS, and linker flags as MY_C_LINKER_FLAGS, then
use them in the Wx OpenGL compile test.
This fixes the test erroneously failing on git mxe on mac.
Also, don't use -fPIC on Win32, set Wx defines separately from the test,
and save all CMAKE_REQUIRED_* variables before the test and restore them
afterwards.
687c08a051