Build: Pass compiler flags from make to setup.py

Extend setup.py to allow environment variables to be set on the
command line. Use that functionality to replace the pkg-config
invocations with flags passed on from make. Suppress the annoying
-Wstrict-prototypes warning by overriding the OPT variable.

Also move the "cd bindings/python" from Makefile.am to setup.py
to side-step problems with "cd" in make rules.

This also fixes bug #628.