waffle: add gbm support

v3:
 * incorporate changes from Chad's code review
 * add enum for gbm platform in a separate commit
 * add gbm support to gl_basic in a separate commit
 * create a separate non-functional 'skeleton' gbm commit

v2:
 * gbm_display_connect: attempt to open specified name before searching
   for devices via libudev.
 * cleanup various wayland references
 * add libudev to cmake

v1:
 * gl_basic appears to function