desktop shell and weston keyboard both refer to themselves prefixed by
LIBEXECDIR, however this is only valid once installed. make check will
currently either fail or run pre-existing versions.
This patch adds a way to override that location by setting the env var
WESTON_BUILD_DIR - which is then set by the test env script so make check
will test the versions in the build directory regardless of whether they're
installed or not.
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
c7210434d4