java_gen: make annoying enum warning a debug message
java_gen.OFMatchV3V13: auto-gen list of supported fields
java_gen.OFOxms: throw exception when user asks for unknown OXM
java_model: remove hand-generated oxm_map
Makefile: fix small bug

Symlink syntax caused java_gen/pre-written/src/src to be created
Merge pull request #133 from andi-bigswitch/master

java_gen.OFMatchV3V13: auto-gen list of supported fields