Bug fix #77 incorrect border_width handling & #73 window data issue

- (Hopefully) fix all incorrect handling of w->a.border_width in compton
  (#77). Thanks to baskerville for reporting.

- Attempt to fix #73 by correcting a mistake that window data is fetched
  from the wrong function. Thanks to zakkak.

- Add git commit/tag detection to Makefile for automatic versioning.

- Change -lGL linking order, to fix a segmentation fault caused by
  something in nvidia-drivers under FreeBSD, mentioned in #74. Thanks
  for the report from DachiChang.

- Link to -levent_core instead of -levent in Makefile. We might move to
  libev soon, though.

- Increase SWOPTI_TOLERANCE to handle the extraordinary delay of
  kqueue() under FreeBSD. Thanks for DachiChang's report.

- Add helper function dump_drawable() for debugging.

- Replace XInternAtom() calls with get_atom().

- Remove -lrt as it's unneeded.