Ensure AUTOBUILD is in native_path form for non-cygwin tools.

Until we can propagate the corresponding buildscripts changes, we must
explicitly put AUTOBUILD in proper form. For now, assume that AUTOBUILD has
not yet been normalized.

build.sh

@@ -216,6 +216,8 @@ fi
 # Check to see if we're skipping the platform
 eval '$build_'"$arch" || pass
 
+# ensure AUTOBUILD is in native path form for child processes
+AUTOBUILD="$(native_path "$AUTOBUILD")"
 # set "$autobuild" to cygwin path form for use locally in this script
 autobuild="$(shell_path "$AUTOBUILD")"
 if [ ! -x "$autobuild" ]