Log Message:
devel/gps: Fix build on SunOS by using COMPILER_RPATH_FLAG

The SunOS linker doesn't understand -rpath, so replace it with
COMPILER_RPATH_FLAG to fix build on SunOS.

Also removing empty directories in the post-install target is not
necessary on pkgsrc.  The SunOS find program doesn't recognize -empty
switch, remove the redundant command to avoid error messages.