GLM 0.9.7.6

Improvements:

• Added pkg-config file #509
• Updated list of compiler versions detected
• Improved C++ 11 STL detection #523

Fixes:

• Fixed STL for C++11 detection on ICC #510
• Fixed missing vec1 overload to length2 and distance2 functions #431
• Fixed long long warnings when using C++98 on GCC and Clang #482
• Fixed scalar reciprocal functions (GTC_reciprocal) #520