It seems to not get past this step:
[ 16%] Building CXX object src/CMakeFiles/hphp_runtime_static.dir/lib/system/gen/sys/dynamic_table_func.no.cpp.o
UPDATE
The above was just slow. I left my desk and came back after several hours, and the build had completed successfully.
There was a similar post on the mailing list but unfortunately it seems the user that hit this problem somehow got past it and got to an actual error message later on in the thread. You might be able to pick up on the thread or contact that person and see if they can detail how they got past this step though.