Support older compilers in HEDLEY_STATIC_ASSERT#65

Implement static assert like funcionality on older compilers, the implementation do not support displaying human redable message, but compilation will fail. Implementation works in any scope, also multiple uses of assert in the same line works fine. Only issue is that it will generate warnings under gcc/clang due to use of unnamed struct with no instances. The way to dissable the warning would be generating struct name using: `struct HEDLEY_CONCAT(_hedley_static_assert_, __COUNTER__) { ... }` under some compilers.