Support fortran compiler

Tried to build extension module which is fortran code, fails. I tried a few variations on the compiler name.

[extension=libforbdf]
sources = libforbdf.f95

[global]
compilers = gfortran

Traceback:

ERROR:root:Error parsing
Traceback (most recent call last):
  File "/home/yhvh/src/third/pybdf/.eggs/pbr-5.2.1-py2.7.egg/pbr/core.py", line 96, in pbr
    attrs = util.cfg_to_args(path, dist.script_args)
  File "/home/yhvh/src/third/pybdf/.eggs/pbr-5.2.1-py2.7.egg/pbr/util.py", line 267, in cfg_to_args
    register_custom_compilers(config)
  File "/home/yhvh/src/third/pybdf/.eggs/pbr-5.2.1-py2.7.egg/pbr/util.py", line 491, in register_custom_compilers
    compiler = resolve_name(compiler)
  File "/home/yhvh/src/third/pybdf/.eggs/pbr-5.2.1-py2.7.egg/pbr/util.py", line 189, in resolve_name
    ret = getattr(ret, part)
UnboundLocalError: local variable 'ret' referenced before assignment