Makefile: Hook MIT KRB5 into the build

Add tests for MK_MITKRB5. If "yes" build MIT KRB5. If "no" build Heimdal.
The default is MK_MITKRB5 = no, added by "krb5: Add build plumbing".

At some point we will change the default to MK_MITKRB5 = yes. A ports
exp-run will need to be successfully run first.

Sponsored by:		The FreeBSD Foundation
Differential revision:	https://reviews.freebsd.org/D50815

Makefile.libcompat

@@ -91,7 +91,11 @@ _LC_LIBDIRS.yes=		lib
 _LC_LIBDIRS.yes+=		gnu/lib
 _LC_LIBDIRS.${MK_CDDL:tl}+=	cddl/lib
 _LC_LIBDIRS.${MK_CRYPT:tl}+=	secure/lib
+.if ${MK_MITKRB5} != "no"
+_LC_LIBDIRS.${MK_KERBEROS:tl}+=	krb5/lib
+.else
 _LC_LIBDIRS.${MK_KERBEROS:tl}+=	kerberos5/lib
+.endif
 
 .if ${MK_FILE} != "no"
 _libmagic=	lib/libmagic