media-video/mpv: enforce proper pkg-config in 9999

media-video/mpv: enforce proper pkg-config in 9999

Export PKG_CONFIG.
Otherwise mpv uses waf machinery to basically do $(which pkg-config)
and then uses the result. This command is probably harmless in most
cases, but it breaks cross-compilation entirely as compiler arguments
are polluted with flags like '-L/usr/lib', which breaks e.g. linking.

Export also CC and AR.
This works fine now, but leave no chances to get screwed by waf.

Package-Manager: Portage-2.3.3, Repoman-2.3.1
Closes: https://github.com/gentoo/gentoo/pull/3718