shells/fish: fix the build with Clang-6.0

Clang-6.0 on FreeBSD 12 doesn't define cpp_lib_make_unique. Patch fish to
key off of the value of cplusplus instead.

PR: 225456
Reported by: pkg-fallout
Reviewed by: jbeich, dim
Differential Revision: https://reviews.freebsd.org/D14058