wafHook: export PKGCONFIG if needed

This is useful for cross-compilation, where nixpkgs sets PKG_CONFIG.
waf decided to use PKGCONFIG instead, so we'll set it when necessary.

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

@@ -10,6 +10,10 @@ wafConfigurePhase() {
         wafConfigureFlags="${prefixKey:---prefix=}$prefix $wafConfigureFlags"
     fi
 
+    if [ -n "${PKG_CONFIG}" ]; then
+      export PKGCONFIG="${PKG_CONFIG}"
+    fi
+
     local flagsArray=(
         "${flagsArray[@]}"
         $wafConfigureFlags "${wafConfigureFlagsArray[@]}"