Do not set QMAKE_{INC,LIB}DIR_{OPENGL,X11} in the freebsd mkspecs.

X11 and the GL libraries are installed into /usr/local like other
software, and this has been the case for many years.

Change-Id: Ied4d9d61154014db3861bdbd6a5bdbe68e76f878
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>