Add Google Test and Google Mock

Enabled with --enable-google-test, configure the location of the sources
with --with-gtest-root, --with-gtest-headers, --with-gmock-root, and
--with-gmock-headers. The latter isn't necessary if the headers are
installed in /usr/include and the sources in /usr/src as the Debian
packages do.

This is a first-pass and needs to be made a bit more sophisticated later.
It might also be worthwhile to extract it into an m4 macro.