fixing C coverage to run with numpy.distutils

When changing to numpy.distutil setup.py generation of c_coverage was
broken, see #562. This is fixed with this commit, by the following steps:
* introduce FFLAGS, CFLAGS do not work for numpy setup.py gfortran
* -lgcov needs to be linked explicitly for gfortran, adapting setup.py
  with an environment variable switch to add gcov
* python setup.py clean --all develop does not clean up *.o *.so files any
  more. Therefore, handling cleanups explicitly in the script (maybe we
  should go for pip)
Merge branch 'master' into 562
Merge pull request #562 from mbyt/562

C Coverage does not work (was: libtau_Linux_32bit_py27.so: undefined symbol: __gcov_merge_add)