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".)
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).
