Only call define versions script in mingwenv after a build is successful

This stops clean.sh script complaining about a non-existant path

tools/windows_packaging/mingw-env.sh

@@ -26,13 +26,13 @@ BUILD_DIR=$BASE/build
 BUILD_CACHE_FILE=$BUILD_DIR/c4che/_cache.py
 TOOLS_DIR=$BASE/tools/windows_packaging
 
-. ../define_versions.sh
-
 APPNAME=`grep -m 1 '^APPNAME' $BASE/wscript | awk '{print $3}' | sed "s/'//g"`
 
 # These are only relevant after a build
 if test -f $BUILD_CACHE_FILE
 then
+        . ../define_versions.sh
+
 	# Figure out the Build Type
 	if grep -q "DEBUG = True" $BUILD_CACHE_FILE; then
 		DEBUG=1