sendmail: Remove runtime dependency on clang

etc/mail/Makefile includes bsd.own.mk, which includes bsd.compiler.mk,
which fails with an error if it can't locate a C compiler. Set
_WITHOUT_SRCCONF=yes before including bsd.own.mk to disable this
behaviour.

Reviewed by: gshapiro
Differential Revision: https://reviews.freebsd.org/D52134

(cherry picked from commit e957c041d7058fcf070323e847db2c0c0479a111)