llvm-cov: also install as gcov (if GNU gcov is disabled)

llvm-cov provides a gcov-compatible interface when invoked as gcov.

Reviewed by:	dim, markj
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D17923

tools/build/mk/OptionalObsoleteFiles.inc

@@ -2461,7 +2461,7 @@ OLD_FILES+=usr/share/man/man1/gperf.1.gz
 OLD_FILES+=usr/share/man/man1/gperf.7.gz
 .endif
 
-.if ${MK_GCC} == no || ${MK_GCOV} == no
+.if (${MK_GCOV} == no || ${MK_GCC} == no) && ${MK_LLVM_COV} == no
 OLD_FILES+=usr/bin/gcov
 OLD_FILES+=usr/share/man/man1/gcov.1.gz
 .endif

usr.bin/clang/llvm-cov/Makefile

@@ -1,6 +1,12 @@
 # $FreeBSD$
 
+.include <src.opts.mk>
+
 PROG_CXX=	llvm-cov
+.if ${MK_GCOV} == "no"
+LINKS=		${BINDIR}/llvm-cov	${BINDIR}/gcov
+MLINKS=		llvm-cov.1		gcov.1
+.endif
 
 SRCDIR=		tools/llvm-cov
 SRCS+=		CodeCoverage.cpp