scons: Create a Configure checker for pkg-config.

scons: Create a Configure checker for pkg-config.

This will check if a pkg-config package exists at all, and then if it
does will attempt to use the supplied pkg-config arguments and
ParseConfig to set up the environment as needed.

Change-Id: I1495e5370b60dcebb1c9ce38517e84d727abc2fd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40866
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>