Fix warnings on hash functions with GCC and -Wnoexcept:
* Add GLM_HAS_NOEXCEPT flag & GLM_NOEXCEPT #define to setup.hpp.
* Add GLM_NOEXCEPT to hash functions in hash.hpp.
* Add GLM_NOEXCEPT to matrix operator[] accessors.
* Add gtx_hash.cpp and a test to verify all hash overloads compile.
Configure with -DCMAKE_CXX_FLAGS="-Werror -Wnoexcept" to test.
