oss-fuzz: Explicitly disable system's glib and PCRE

They were not used because oss-fuzz didn't have those libraries
installed. Installing other packages there might pull them in as
dependencies. To avoid surprises, explicitly disable them.

Change-Id: I025744587e9907af71541b2a43eed982111aef42
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>
Reviewed-by: Tim Blechmann <tim.blechmann@qt.io>