qgroundcontrol: Fix running of {pre,post}Configure

Commit 0055c6a introduced a new preConfigure hook that sets the right
qmake path. Unfortunately the mkDerivation attributes of qgroundcontrol
override the whole configurePhase, so this hook isn't run at all.

Instead of using pushd/popd, I'm making it a bit more readable by just
spawning a subshell with the right working directory.

This fixes the build of qgroundcontrol and it now successfully compiles
on my machine.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/applications/science/robotics/qgroundcontrol/default.nix

@@ -29,12 +29,10 @@ stdenv.mkDerivation rec {
   patches = [ ./0001-fix-gcc-cmath-namespace-issues.patch ];
 
   configurePhase = ''
+    runHook preConfigure
     mkdir build
-    pushd build
-
-    qmake ../qgroundcontrol.pro
-
-    popd
+    (cd build && qmake ../qgroundcontrol.pro)
+    runHook postConfigure
   '';
 
   preBuild = "pushd build/";