Export PYTHONPATH from the Makefile.

[gsmcmullin]

Fixes this non-descriptive problem when running make:

Traceback (most recent call last):
  File "sbpg/generator.py", line 20, in <module>
    import sbpg.specs.yaml2 as yaml
ImportError: No module named sbpg.specs.yaml2

[swiftnav-jenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
http://zazu.ci.swiftnav.com/job/libsbp_Pull_Requests/87/
Test PASSed.

[fnoble]

I think there is a better way to fix this, I think its a symptom of not using packages correctly. Will take a look.

[gsmcmullin]

I'm merging this since nobody's offered a better solution.