Upstream provides pkgconfig files for quite some time now [1,2]. Use them to determine the required flags instead of hard coding. This makes cross-compilation easy, which I dare to say is important for many raspberry-pi users. This also prevents picking libEGL and libGLESv2 from mesa when they are present, which can happen with the current code. Good distros should put these pkgconfig files into default pkg-config search path or populate PKG_CONFIG_PATH for users. However, be nice to everybody and manually look into '/opt/vc/lib/pkgconfig' just in case. Hence the PKG_CONFIG_PATH mangling. [1]: https://github.com/raspberrypi/userland/issues/245 [2]: https://github.com/raspberrypi/userland/commit/05d60a01d53dca363bb4286594db1826ffff8762