cmake: enable parallel building of cmake projects by default

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

···
 
     cmake ${cmakeDir:-.} $cmakeFlags "${cmakeFlagsArray[@]}"
 
+    if ! [[ -v enableParallelBuilding ]]; then
+        enableParallelBuilding=1
+        echo "cmake: enabled parallel building"
+    fi
+
     runHook postConfigure
 }