wafHook: always enable parallel building (#136641)

Follow the same pattern as used in the cmake setup hook to assume
`enableParallelBuilding=1` when unset by the user. This makes wafHook
honor `NIX_BUILD_CORES`. See #136368 and #136641.

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

@@ -21,6 +21,11 @@ wafConfigurePhase() {
     echoCmd 'configure flags' "${flagsArray[@]}"
     python "$wafPath" "${flagsArray[@]}"
 
+    if ! [[ -v enableParallelBuilding ]]; then
+        enableParallelBuilding=1
+        echo "waf: enabled parallel building"
+    fi
+
     runHook postConfigure
 }