MacOSX: allow build with homebrew and its emulated MesaGL

[dk]

This change comes from Prima::OpenGL that is build with Prima that uses lots of homebrew libraries, and is actually advised to be build on homebrew. The problem comes with the libraries that are implicitly dependant on the native /opt/X11 libraries, and any program that is compiled with both set of Xlibs (and by extention, GL libs), dumps core.

A possibile solution to this is to let OpenGL::Modern to use homebrew libraries, but that comes with a price, native mac gl will not be used, but the slow emulated MesaGL will be used instead.

[coveralls]

Pull Request Test Coverage Report for Build 15029398118

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.237%

---

Totals
Change from base Build 14850068369:	0.0%
Covered Lines:	4485
Relevant Lines:	7324

---

💛 - Coveralls

[mohawk2]

Thank you for this PR. Could you rebase it past latest master to resolve conflicts so I can take a proper look?

[dk]

rebased, please take a look