When --enable-strict-locations is used, the location of Xorg modules is
not autodetected. Configuration files are installed under sysconfdir.
Don't make any logic depend on the presence of the --prefix option, it's
not an expected side effect.
Use --enable-strict-locations in "make distcheck", it's needed to make
sure all files are installed under prefix.
tests/xorg-test-run.sh does not modify anything outside the build
directory. It doesn't need root access to install the config file.
Distribute all files in tests directory, including xorg-test-run.sh
