Change PKGSRC_USE_SSP to a quad option.  The options are now:

  "no"    Do not pass any stack protection flags (the default)
  "yes"    Pass -fstack-protector
  "strong" Pass -fstack-protector-strong
  "all"    Pass -fstack-protector-all

This allows users to configure the level of stack smashing protection they
require, and ensures consistent behaviour across platforms.  Users running
on NetBSD who previously used the option will need to change "yes" to "all"
to match the previous default configuration for that platform.

While here use _WRAP_EXTRA_ARGS to ensure the flag gets passed regardless
of whether the package honours CFLAGS, and support additional wrappers.

Discussed a while back with khorben, and used in production for the SmartOS
2016Q1 package sets with the "strong" option.

(jperkin)