Add directive to force default constructors

[patnashev]

If constructors and assignment operator are defined as default:

GLM_FUNC_DECL tvec2() = default;
GLM_FUNC_DECL tvec2(tvec2<T, P> const & v) = default;
GLM_FUNC_DECL tvec2<T, P>& operator=(tvec2<T, P> const & v) = default;

then tvec2 can be used in unions. Implicitly-defined constructors provide no functionality but make the classes non-trivial.

[plasmacel]

it's not a bad idea

[Groovounet]

I agree, I like it.

[Groovounet]

This feature has been in master branch for GLM 0.9.7 release.

Thanks,
Christophe