waf: backport finding of pkg-config

waf: backport finding of pkg-config

Allow the builder to customize the location of pkg-config
utility by setting PKGCONFIG environment variable.

This is backported from upstream waf.

Thanks to Zentaro Kavanagh <zentaro@google.com> for
pointing that out and proposing the fix.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12529

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Jan 25 04:23:00 CET 2017 on sn-devel-144