shells/zsh: Fix runtime error when built by gcc-5 with upstream patch

ZSH was misbuilt on gcc-5 due to an intentional change in behavior of
gcc5's cpp. This patch to execute cpp with the -P switch comes from
upstream. No revbump necessary as GCC5 isn't used by default on FreeBSD.

Reported by: DragonFly ZSH users
Approved by: maintainer (bapt, IRC)