meson: enable parallel building of meson projects by default

pkgs/development/tools/build-managers/meson/setup-hook.sh

···
     meson build $mesonFlags "${mesonFlagsArray[@]}"
     cd build
 
+    if ! [[ -v enableParallelBuilding ]]; then
+        enableParallelBuilding=1
+        echo "meson: enabled parallel building"
+    fi
+
     runHook postConfigure
 }