Still not quite ready to call this solved even though I got the "why"...

f2py will let LDFLAGS override the default built-in linker flags from "linker_exe" and "linker_so". I would think at a minimum you would want it as an append, not an override if not outright ignore the environment variables for this specific case.

$ f2py3 -c --help-fcompiler
Gnu95FCompiler instance properties:
[SNIP]
  linker_exe      = ['/usr/bin/gfortran', '-Wall', '-Wall']
  linker_so       = ['/usr/bin/gfortran', '-Wall', '-Wall', '-shared']

Thanks,
Richard