[#68] Moved the server build.properties file check to top level in build.xml.
This will catch a missing properties file problem instead of misleadingly reporting an error when importing the Ant macros.
[#68] Moved external build.properties linking back into compile_server.sh script.
This optional setup is mainly used for builds on the CI server.
Removed build mode selection since properties linking could also be used for non-CI builds.