Mk/Uses/pkgconfig.mk: add support for PKGCONFIG_PATHS

USES=pkgconfig now allows you to specify PKGCONFIG_PATHS to allow
management of PKG_CONFIG_PATHS in a cooperative fashion.

PR: 260960
Reported by: Martin Neubauer <m.ne@gmx.net>
Approved by: bofh
Differential Revision: https://reviews.freebsd.org/D40194