improve build script to not "override" values set by others in their local builds.

releng.control/commonVariations.shsource

@@ -120,7 +120,7 @@ export BUILD_CVS_ROOT=${BUILD_CVS_ROOT:+/cvsroot/webtools}
 
 # for  builds on local machines, SKIPUSERS should be 'true' to avoid
 # notifying users about those builds on your local machine
-export SKIPUSERS=${:+false}
+export SKIPUSERS=${SKIPUSERS:+false}
 
 export DOWNLOAD_ROOT=${DOWNLOAD_ROOT:+/home/data/httpd/download.eclipse.org}