automake: rework VL dependency tracking

automake: rework VL dependency tracking

Set a single VL_{CFLAG,LIBS} for xcb and friends, and let each target
check for it's relevant library alone. Required as with follow up
commits we'll build aux/vl into a separate module, which needs VL_CFLAGS

Cleanup add a couple of explicit LIBDRM_LIBS linking, as aux/vl itself
requires libdrm, despite that LIBDRM_{RADEON,NOUVEAU...} may provide it
as well.

v2: Rebase. Make sure st/xvmc programs work.

Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>