gst-plugins-base: make sure alsa-lib is detected if present

Make sure alsa-lib is built before gst-plugins-base if enabled, so
configure detects the presence of alsa and builds alsasink/src.

Reported-by: Phil Edworthy <Phil.Edworthy@renesas.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>