MAINT: runtests: add option for gcov

Recent GCC versions apparently support gcov also in shared libraries. This makes it fairly simple to get C code coverage information for Numpy.

This PR adds options `--gcov` and `--lcov-html` to generate code coverage info also for the C code in Scipy.
The lcov tool is required for pretty HTML output (has packages at least for Debian/Ubuntu).

Unfortunately, `gcov` doesn't seem to produce traces for the Fortran files for some reason, but C/C++/Cython is still good.

Seeing how much of the C code is covered by the test suite is as simple as
```
python runtests.py --gcov
python runtests.py --lcov-html
firefox build/lcov/index.html
```
The hard-coded GCC compiler flags should be fine on most Linux platforms at the least.