qlcplus: fix gcc-13 build (drop -Werror)

Without the change build fails as
https://hydra.nixos.org/build/250969590:

    chaserrunner.cpp: In member function 'bool ChaserRunner::write(MasterTimer*, QList<Universe*>)':
    chaserrunner.cpp:765:36: error: pointer used after 'void operator delete(void*, std::size_t)' [-Werror=use-after-free]
      765 |             m_runnerSteps.removeOne(step);
          |             ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~

pkgs/applications/misc/qlcplus/default.nix

@@ -29,16 +29,20 @@ mkDerivation rec {
 
   qmakeFlags = [ "INSTALLROOT=$(out)" ];
 
-  env.NIX_CFLAGS_COMPILE = "-Wno-error=deprecated-declarations";
-
   postPatch = ''
     patchShebangs .
     sed -i -e '/unix:!macx:INSTALLROOT += \/usr/d' \
             -e "s@\$\$LIBSDIR/qt4/plugins@''${qtPluginPrefix}@" \
             -e "s@/etc/udev/rules.d@''${out}/lib/udev/rules.d@" \
       variables.pri
+
+    # Fix gcc-13 build failure by removing blanket -Werror.
+    fgrep Werror variables.pri
+    substituteInPlace variables.pri --replace-fail "QMAKE_CXXFLAGS += -Werror" ""
   '';
 
+  enableParallelBuilding = true;
+
   postInstall = ''
     ln -sf $out/lib/*/libqlcplus* $out/lib
   '';