cmake: Only enable C++11 if enable_if is supported

This is to avoid putting the compiler into C++11 mode on MacOS 10.8 which uses a broken hybrid of LLVM/clang with the GNU GCC libstdc++ standard library by default.  This breaks things when the compiler supports C++11 but does not have the necessary functional library features.

Testing: This is actually for OmeroCpp which sources this file.  Build on MacOS 10.8 with Ice 3.5.  This will be tested by http://ci.openmicroscopy.org/view/5.1/job/OMERO-5.1-merge-cpp/ (marlin).  It's expected to work on all other platforms/Ice version with or without this patch.  The only reason this combination is that Ice 3.5 conditionally enables C++11 features which the MacOS 10.8 toolchain dislikes for the reason above.
 
--no-rebase