Improvements:

• Added Visual C++ 15 detection
• Added Clang 4.0 detection
• Added warning messages when using GLM_FORCE_CXX** but the compiler
  is known to not fully support the requested C++ version #555
• Refactored GLM_COMPILER_VC values
• Made quat, vec, mat type component length() static #565

Fixes:

• Fixed Visual C++ constexpr build error #555, #556