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

etc/mail/Makefile

@@ -241,5 +241,7 @@ restart restart-mta restart-mspq:
 .include "Makefile.local"
 .endif
 
-# For the definition of $SHAREMODE
+# For the definition of $SHAREMODE.  Define _WITHOUT_SRCCONF to prevent this
+# including <bsd.compiler.mk>, which requires clang.
+_WITHOUT_SRCCONF=yes
 .include <bsd.own.mk>