Fix user-defined literal detection for Intel C++ compiler (#311)

[dean0x7d]

Fixes #311.

The FMT_USE_USER_DEFINED_LITERALS check is beginning to resemble a dumpster fire, but as per Intel's C++11 feature notes both the system compiler (gcc/msc) and icc itself must support the given feature:

On Windows: when using Intel C++ compiler with Visual Studio 2010* or 2012*, the C++11 features supported by Visual C++ 2010/2012 are enabled by default. Use "/Qstd=c++11" to turn on the support for all other cases.
On Linux or Mac OS X: the C++11 features supported by gcc on the path are enabled by default. Use "-std=c++11" to turn on the support for all other cases.

To make sure the UDL detection still works on any compiler I added a new compile test which checks that the feature detection in the header matches the one in CMake. CMAKE_CXX_COMPILER and CPP11_FLAG are passed to compile-test to ensure the same configuration.

Tested and working with Intel C++ 14.0-16.0 on Linux and 16.0 on Windows.

[vitaut]

Merged, thanks a lot for fixing this!