Merge pull request #16 from dkg/master

minor updates, referenced from the mailing list.
Avoid unnecessary double linking
include the CPPFLAGS variable for C preprocessor flags
create a way to build xdotool without setting rpath (see e.g. https://wiki.debian.org/RpathIssue)
provide a way to override which specific test combinations are run

This is lets packagers indicate which particular test runs are desired
(e.g. so that build daemons don't have to have gnome-session installed
but can still run some of the tests with a lighter-weight environment)