wafHook: make crossFlags optional

The convention of `--cross-compile` and `--cross-execute` is common
enough that it seems like a reasonable default. However there are
projects like mpv which do not use these flags, and rightfully fails
to configure when passed unexpected flags.

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

@@ -11,11 +11,13 @@ wafConfigurePhase() {
     fi
 
     local flagsArray=(
-        @crossFlags@
         "${flagsArray[@]}"
         $wafConfigureFlags "${wafConfigureFlagsArray[@]}"
         ${configureTargets:-configure}
     )
+    if [ -z "${dontAddWafCrossFlags:-}" ]; then
+        flagsArray+=(@crossFlags@)
+    fi
     echoCmd 'configure flags' "${flagsArray[@]}"
     python "$wafPath" "${flagsArray[@]}"