Remove empty generated file upon gperf failure

Prior to this change the build could fail as follows, if gperf is not
available (or fails):

  - make(1) stops due to the gperf error, but an empty target file
    (cfns.h) is still created
  - the empty cfns.h is newer than the source cfns.gperf so it is not
    regenerated on subsequent builds
  - the gcc build fails (undefined reference to libc_name_p)

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days

gnu/usr.bin/cc/cc1plus/Makefile

@@ -30,7 +30,7 @@ LDADD=	${LIBBACKEND} ${LIBCPP} ${LIBDECNUMBER} ${LIBIBERTY}
 # C++ parser
 cfns.h: cfns.gperf
 	gperf -o -C -E -k '1-6,$$' -j1 -D -N 'libc_name_p' -L ANSI-C \
-	    ${.ALLSRC} > ${.TARGET}
+	    ${.ALLSRC} > ${.TARGET} || (rm -f ${.TARGET}; false)
 CLEANFILES= cfns.h
 
 DOBJS+=	${SRCS:N*.h:R:S/$/.o/g}