Use pkgconfig to get flags needed for GL library.

[falsifian]

This gets it to build on OpenBSD. Before this change, the GL library isn't
found:

Configuring library for OpenGLRaw-3.3.4.0..
cabal: Missing dependency on a foreign library:
* Missing (or bad) C library: GL

[falsifian]

The GL library is under /usr/X11R6 on OpenBSD. My workaround for building something that depends on OpenGLRaw for now is: C_INCLUDE_PATH=/usr/X11R6/include LIBRARY_PATH=/usr/X11R6/lib cabal v2-install ..

[svenpanne]

I think this will break compilation under Windows and all other platforms without pkg-config (not very likely for *nix variants, but nevertheless). The logic in that part of the .cabal file is very delicate and took a long time to get (mostly) right, so I'm very conservative about changes there. Could you guard your change with an if os(openbsd)?

[falsifian]

Done, and added a comment. Let me know if you'd like me to move it somewhere else or adjust or edit the comment.

I wonder why the build works on other systems. Maybe Cabal has /usr/include and /usr/lib hard-coded.