Fix compilation on cygwin after commit 762c9766c93697af8d7fbaa729aed118789dbe8e

Fix compilation on cygwin after commit 762c9766c93697af8d7fbaa729aed118789dbe8e

Fix compilation on cygwin after commit 762c9766c93697af8d7fbaa729aed118789dbe8e
"Use VERT_ATTRIB_* indexed array in gl_array_object" added the first non-driver
use of ffsll(), which exposes the fact that this isn't provided on cygwin.

Found by tinderbox, see [1]

[1] http://tinderbox.freedesktop.org/builds/2011-11-30-0017/logs/libGL/#build

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Brian Paul <brianp@vmware.com>