qt54: add missing mesa include dir

Try to build e.g. the Qt5 Camera Example[1] and see that qmake fails to
find <GL/gl.h>. This fixes it.

[1] http://doc.qt.io/qt-5/qtmultimediawidgets-camera-example.html
(Although since nixpkgs qtcreator still lacks 'examples', we have to
download the sources manually and use "qmake && make".)

pkgs/development/libraries/qt-5/5.4/0014-mkspecs-libgl.patch

@@ -4,11 +4,13 @@ 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 @@
+@@ -12,8 +12,8 @@
+ QMAKE_LIBDIR            =
  QMAKE_INCDIR_X11        =
  QMAKE_LIBDIR_X11        =
- QMAKE_INCDIR_OPENGL     =
+-QMAKE_INCDIR_OPENGL     =
 -QMAKE_LIBDIR_OPENGL     =
++QMAKE_INCDIR_OPENGL     = @mesa@/include
 +QMAKE_LIBDIR_OPENGL     = @mesa@/lib
  QMAKE_INCDIR_OPENGL_ES2 = $$QMAKE_INCDIR_OPENGL
  QMAKE_LIBDIR_OPENGL_ES2 = $$QMAKE_LIBDIR_OPENGL