GCC: Fix noexcept warnings on hash functions

[genpfault]

Fixes warnings with the hash functions when building with -Wnoexcept on GCC:

// g++ -Wsystem-headers -Wnoexcept main.cpp
#include "glm/glm.hpp"
#define GLM_ENABLE_EXPERIMENTAL
#include "glm/gtx/hash.hpp"

#include <unordered_map>

int main()
{
    std::unordered_map<glm::ivec2, int> foo;
    foo[glm::ivec2(0, 0)]++;
    
    return 0;
}

Output:

In file included from glm/detail/setup.hpp:701,
                 from glm/glm.hpp:105,
                 from main.cpp:2:
/usr/include/c++/10/type_traits:2915:14: warning: noexcept-expression evaluates to ‘false’ because of a call to ‘std::size_t std::hash<glm::vec<2, T, Q> >::operator()(const glm::vec<2, T, Q>&) const [with T = int; glm::qualifier Q = glm::packed_highp; std::size_t = long unsigned int]’ [-Wnoexcept]
 2915 |       return noexcept(std::declval<_Fn>()(std::declval<_Args>()...));
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from glm/gtx/hash.hpp:142,
                 from main.cpp:4:
glm/gtx/hash.inl:32:28: note: but ‘std::size_t std::hash<glm::vec<2, T, Q> >::operator()(const glm::vec<2, T, Q>&) const [with T = int; glm::qualifier Q = glm::packed_highp; std::size_t = long unsigned int]’ does not throw; perhaps it should be declared ‘noexcept’
   32 |  GLM_FUNC_QUALIFIER size_t hash<glm::vec<2, T, Q>>::operator()(glm::vec<2, T, Q> const& v) const
      |     

GCC version:

$ g++ --version
g++ (Debian 10.2.1-6) 10.2.1 20210110

[genpfault]

StoleBorrowed the GLM_NOEXCEPT idea from JesseTG's 2015 PR.

[christophe-lunarg]

This looks like a very reasonable change with tests, thanks !