configure: don't override pkg-config if there's one already set
configure: don't override pkg-config if there's one already set

I wrote a wrapper pkg-config script for a specific build but
grilo-pluins overrides my $PKG_CONFIG environment variable so my
script isn't used.

Signed-off-by: Jonny Lamb <jonnylamb@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=695789

MathieuDuponchelle/grilo-plugins,jasuarez/grilo-plugins,kyoushuu/grilo-plugins,MikePetullo/grilo-plugins,grilofw/grilo-plugins,MathieuDuponchelle/grilo-plugins,jasuarez/grilo-plugins,vrutkovs/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins,kyoushuu/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,vrutkovs/grilo-plugins,kyoushuu/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,GNOME/grilo-plugins