qtbase: use more than one core to build qmake

The qtbase package's `configurePhase` builds the `qmake` tool.  In the
current nixpkgs expression it does this single-threadedly, by invoking
`make` without a `-j` flag.

Let's thread `NIX_BUILD_CORES` through to `./configure`'s invocation
of `make` via the `MAKEFLAGS` variable.

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

pkgs/development/libraries/qt-5/modules/qtbase.nix

@@ -151,6 +151,9 @@ stdenv.mkDerivation {
     ''}
 
     NIX_CFLAGS_COMPILE+=" -DNIXPKGS_QT_PLUGIN_PREFIX=\"$qtPluginPrefix\""
+
+    # paralellize compilation of qtmake, which happens within ./configure
+    export MAKEFLAGS+=" -j$NIX_BUILD_CORES"
   '' + lib.optionalString (compareVersion "5.15.0" >= 0) ''
     ./bin/syncqt.pl -version $version
   '';