Check if test suite should be built

The test suite is built by default. It would be nice if it was only
built by default when the source directory was checked out of version
control. There's no bullet proof way to check for this, but it could be
assumed that's the case when the source directory contains a .git
directory. The user can still decide if the test suite should be built
or not using the option.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>