meson: conditionally include -ldl in gbm pkg-config file

Follow libGL and only include -ldl in gbm pkg-config file if libdl was
actually found.  Many systems have these functions in libc and don't
have libdl.

Fixes: 816bf7d1644 ("meson: build gbm")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5630>
2 files changed