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

Makefile.inc1

@@ -676,7 +676,7 @@ BSARGS= 	DESTDIR= \
 		MK_HTML=no NO_LINT=yes MK_MAN=no \
 		-DNO_PIC MK_PROFILE=no -DNO_SHARED \
 		-DNO_CPU_CFLAGS MK_WARNS=no MK_CTF=no \
-		MK_CLANG_EXTRAS=no MK_CLANG_FULL=no \
+		MK_CLANG_EXTRAS=no MK_CLANG_FORMAT=no MK_CLANG_FULL=no \
 		MK_LLDB=no MK_RETPOLINE=no MK_TESTS=no \
 		MK_INCLUDES=yes
 
@@ -697,7 +697,7 @@ TMAKE=		\
 		SSP_CFLAGS= \
 		-DNO_LINT \
 		-DNO_CPU_CFLAGS MK_WARNS=no MK_CTF=no \
-		MK_CLANG_EXTRAS=no MK_CLANG_FULL=no \
+		MK_CLANG_EXTRAS=no MK_CLANG_FORMAT=no MK_CLANG_FULL=no \
 		MK_LLDB=no MK_RETPOLINE=no MK_TESTS=no
 
 # cross-tools stage
@@ -2577,6 +2577,7 @@ NXBMAKEARGS+= \
 	SSP_CFLAGS= \
 	MK_CASPER=no \
 	MK_CLANG_EXTRAS=no \
+	MK_CLANG_FORMAT=no \
 	MK_CLANG_FULL=no \
 	MK_CTF=no \
 	MK_DEBUG_FILES=no \