MAINT: runtests: enable generation of C code coverage info via 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 Numpy.
The [lcov](http://ltp.sourceforge.net/coverage/lcov.php) tool is required for pretty HTML output (has packages at least for Debian/Ubuntu).

Seeing how much of the C and Python code is covered by the test suite is as simple as
```
python runtests.py --gcov --coverage
python runtests.py --lcov-html
firefox build/lcov/index.html build/coverage/index.html
```
The hard-coded GCC compiler flags should be fine on most Linux platforms at the least.