Add WITH_CLANG_FORMAT option

clang-format is enabled conditional on either WITH_CLANG_EXTRAS or
WITH_CLANG_FORMAT. Some sources in libclang are build conditional on
either rule, and obviously the clang-format binary itself depends on the
rule.

clang-format could still use a manual page.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D25427