Build and install a shared libLLVM, llvm-config and llvm includes.

This is required to build the radeonsi Mesa driver.
ok patrick@

src/include/Makefile

@@ -1,4 +1,4 @@
-#	$OpenBSD: Makefile,v 1.223 2019/02/04 23:23:18 jsg Exp $
+#	$OpenBSD: Makefile,v 1.224 2019/03/05 09:44:38 jsg Exp $
 #	$NetBSD: Makefile,v 1.59 1996/05/15 21:36:43 jtc Exp $
 
 #	@(#)Makefile	5.45.1.1 (Berkeley) 5/6/91
@@ -61,7 +61,8 @@ RDIRS+= ../gnu/usr.bin/cc/libobjc
 PRDIRS+= ../gnu/lib/libstdc++-v3 ../gnu/usr.bin/cc/include
 .endif
 .if ${BUILD_CLANG:L} == "yes"
-RDIRS+= ../lib/libcxxabi ../lib/libcxx
+RDIRS+= ../lib/libcxxabi ../lib/libcxx \
+	 ../gnu/usr.bin/clang/libLLVM
 .endif
 
 # prereq implies includes