qt5: embed path to mesa (libGL) in Qt mkspecs file

Fixes this problem, when building apps in QtCreator:

  ...(compile output window)
  g++ -Wl,-rpath,/nix/store/1w7h7p6s2srfw2ady90k7072991lrnpp-qtbase-5.4.2/lib \
      -o qt-test3 main.o mainwindow.o moc_mainwindow.o \
      -L/nix/store/1w7h7p6s2srfw2ady90k7072991lrnpp-qtbase-5.4.2/lib \
      -lQt5Widgets -lQt5Gui -lQt5Core -lGL -lpthread
  /nix/store/b8qhjrwf8sf9ggkjxqqav7f1m6w83bh0-binutils-2.23.1/bin/ld: cannot find -lGL
  collect2: error: ld returned 1 exit status

mesa is already in the closure of Qt, so there is no size increase.
The patch is copied into both qt-5.3 and qt-5.4 directories, like other
patches are.

Note that programs still can _run_ against a different libGL (e.g. one
provided by nvidia) by configuring the dynamic linker. For instance,
NixOS sets the LD_LIBRARY_PATH environment variable to
/run/opengl-driver/lib/, meaning that whatever libGL is found there
will be used instead of the default (mesa).