CMake Nightly Date Stamp
FindOpenGL: simplify OS selection code

This was using nested if's, now it uses elseif to flatten that. It also removes
one "if" from the general "else" branch that checks for Apple, as that has it's
own branch anyway and can't be true at this point.
FindOpenGL: require headers to be found on non-Windows platforms (#13746)
Merge topic 'OpenGL-require-includes' into next

5be3593 FindOpenGL: require headers to be found on non-Windows platforms (#13746)
b10a171 FindOpenGL: simplify OS selection code
f2958ff CMake Nightly Date Stamp