Congratz to the git push! 
Consider writing the commit message like "lammps: ...", so it's directly clear from the first word that this is about lammps.

I am not an fortran user (I just need fortran to build numpy/scipy).
Usually, fortran knows about it's own library and location, but the linking is probably done by cc or ld here.

Probably these LDFLAGS should only be added if the brewed gfortran is used. 
Something like wrapping this line in `if ENV['FC'] == HOMEBREW_PREFIX/"bin/gfortran"` (assuming that is the value is right).

FFFLAGS and FCFLAGS are used and passed to the FC but not used for the linker.
This thread is related: https://github.com/mxcl/homebrew/issues/8539