math/blas tries to build lang/gcc7

[Marc Baudoin <babafou%babafou.eu.org@localhost>]

Hi,

I'm trying to build math/blas on NetBSD/amd64 9.1.  As a
dependency, math/blas tries to build lang/gcc7:

[...]
===> Installing dependencies for blas-3.9.0nb1
=> Tool dependency cmake>=2.8.1nb1: found cmake-3.18.4
=> Build dependency cwrappers>=20150314: found cwrappers-20180325
=> Full dependency gcc7>=7.0: NOT found
=> Verifying package-install for ../../lang/gcc7
[...]

althoug NetBSD 9.1 already comes with gcc 7.5.0 in the base
system.  Shouldn't that one be used instead?