Ensure Qt knows where libGL is.

Author: Bjørn Forsman <bjorn.forsman@gmail.com>
diff -uNr qt-everywhere-opensource-src-5.3.2.orig/qtbase/mkspecs/common/linux.conf qt-everywhere-opensource-src-5.3.2/qtbase/mkspecs/common/linux.conf
--- qt-everywhere-opensource-src-5.3.2.orig/qtbase/mkspecs/common/linux.conf	2014-09-11 12:48:07.000000000 +0200
+++ qt-everywhere-opensource-src-5.3.2/qtbase/mkspecs/common/linux.conf	2015-08-23 13:03:30.617473019 +0200
@@ -13,7 +13,7 @@
 QMAKE_INCDIR_X11        =
 QMAKE_LIBDIR_X11        =
 QMAKE_INCDIR_OPENGL     =
-QMAKE_LIBDIR_OPENGL     =
+QMAKE_LIBDIR_OPENGL     = @mesa@/lib
 QMAKE_INCDIR_OPENGL_ES2 = $$QMAKE_INCDIR_OPENGL
 QMAKE_LIBDIR_OPENGL_ES2 = $$QMAKE_LIBDIR_OPENGL
 QMAKE_INCDIR_EGL        =