cmd/go: skip TestCgoPkgConfig if pkg-config is too old

cmd/go: skip TestCgoPkgConfig if pkg-config is too old

pkg-config 0.24 adds support for quoting and escaping whitespace;
distros like CentOS 6 are still shipping pkg-config 0.23. Skip the test
there since there's no way to get whitespace into the pkg-config output.

Fixes #17846.

Change-Id: Ie4ea17e9b709372a20178b539498929754bcd51f
Reviewed-on: https://go-review.googlesource.com/33027
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>