thunkderbird build failure on 8.1-STABLE

[Mike Pumford <mpumford%mudcovered.org.uk@localhost>]

Don't think this is a thunderbird issue I think its actually a gcc 6 
issue which is thunderbird dependency on 8.x.

ERROR: This package has set PKG_FAIL_REASON:
ERROR: [pkg-build-options.mk] This file may only be included from a 
buildlink3.mk file (pkgbase=gcc6).
*** Error code 1

As far as I can tell gcc6 IS including pkg-build-options.mk only from 
its buildlink3.mk file so I'm not really sure why this error is being 
set. I'm seein the same failure on both i386 and amd64 with the same 
pkgsrc tree. 9.0 doesn't hit this but I think that's because the system 
compiler version is sufficient.

Mike