Turn off -Werror for gcc 4.2.1

As part of marching gcc 4.2.1 out of the tree, turn off -Werror on gcc 4.2.1
compiles by default. It generates too many false positives and breaks CI
for no benefit.

Discussed on: arch@
Reviewed by: jhb@, emaste@, pfg@
Differential Revision: https://reviews.freebsd.org/D21378