libqtxdg: fix pkgconfig files

Without this change pkgconfig files contain incorrect values for libdir and
includedir in the form of:

prefix: /nix/store/...liblibqtxdg
libdir: ${prefix}//nix/store/.../libqtxdg/lib
includedir: ${prefix}//nix/store/.../libqtxdg/include
(cherry picked from commit a1fec88)

pkgs/desktops/lxqt/base/libqtxdg/default.nix

@@ -16,7 +16,11 @@ stdenv.mkDerivation rec {
   buildInputs = [ qt5.qtbase qt5.qtsvg ];
 
   preConfigure = ''
-    cmakeFlags+=" -DQTXDGX_ICONENGINEPLUGIN_INSTALL_PATH=$out/$qtPluginPrefix"
+    cmakeFlagsArray+=(
+    "-DQTXDGX_ICONENGINEPLUGIN_INSTALL_PATH=$out/$qtPluginPrefix"
+    "-DCMAKE_INSTALL_INCLUDEDIR=include"
+    "-DCMAKE_INSTALL_LIBDIR=lib"
+    )
   '';
 
   meta = with stdenv.lib; {