Disable -Werror to fix build on versions older than 11.0.

[jhb]

Summary

The Makefiles in this port use CWARNFLAGS.clang to disable certain
warnings on clang. However, the bsd.*.mk files only support this
feature in 11.0 and later. Disable -Werror on older OS versions as
a workaround.

Test Plan

• poudriere testport on 10.3, 11.0, 11.1 and 12.

[jhb]

Committed in rP447184.