Allow coverage with clang.

Note that this doesn't play nicely with acov out of the box. Clang
apparently generates .gcno files that aren't compatible with gcov-4.8.
This can be solved by installing gcc-4.6 and invoking lcov with
`--gcov-tool /usr/bin/gcov-4.6`.

http://stackoverflow.com/questions/17758126/clang-code-coverage-invalid-output

Change-Id: I79547e1c579fa79db47ff07d5e90c42cedbd5cbb