Recommend the use of GCC 7 or later over this port, since that is now
the default version of GCC for the Ports Collection.

And backport r478722 | gerald | 2018-09-01 from lang/gcc7: [1]

Disable the build/use of libssp/gets-chk since FreeBSD 12 and later
do not feature gets() any longer.

PR: 222796, 231066 [1]
Differential Revision: https://reviews.freebsd.org/D12298 [1]