libglib2: use system pcre when available

By default, glib builds its own internal copy of the pcre library, but
it also allows to use the one available from the system. Use this
possibility when pcre is already selected in the Buildroot
configuration.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>