mail/mailsync: Unbreak in 11.4 and 12.2

This port is broken in 12.2 and 11.4, but not in 12.1 or 13-current. The problem
is the old mail/cclient doing things like redefining C++ keywords.

Workaround this by building with GCC in those releases where it is broken.

PR: 250740
Submitted by: colin@fbug.ksac.uk