Using constexpr for constant functions.

[suikki]

It would be nice if glm would use the constexpr keyword more.

For example constats:

    template <typename genType>
    GLM_FUNC_DECL genType pi();

Would instead be:

    template <typename genType>
    GLM_FUNC_DECL GLM_CONSTEXPR genType pi();

Also some functions like degrees() and radians() should use it too.

It would allow using glm constants and many functions like degrees to radians conversion in users own constexpr functions.

It would make sense to add constexpr to many more places too, like some vector and matrix constructors and math operations. It would make possible to move some matrix/vector math to compile time.

[JesseTG]

To really take advantage of this, C++14 has to be fully implemented in all the major compilers; C++11 heavily restricts what constexpr functions can do, but C++14 doesn't.

[Groovounet]

This change has been made in master branch for GLM 0.9.7.1 release.
constexpr coverage could still be improved but that will be for later.

Thanks for contributing,
Christophe