Switch ntp's embedded libevent to 2.1.18

For OpenSSL 1.1.1 compatibility.

In Makefile.inc1 add (to the existing similar cases) a hack to handle
dependencies across the migration.

Reviewed by:	jhb
Approved by:	re (gjb)
Sponsored by:	The FreeBSD Foundation.
Differential Revision:	https://reviews.freebsd.org/D17481

Makefile.inc1

@@ -939,6 +939,13 @@ _cleanobj_fast_depend_hack: .PHONY
 		    ${LIBCOMPAT:D${LIBCOMPAT_OBJTOP}/lib/libc/.depend.${f}.*}; \
 	fi
 .endfor
+# 20181009 track migration from ntp's embedded libevent to updated one
+	@if [ -e "${OBJTOP}/usr.sbin/ntp/libntpevent/.depend.bufferevent_openssl.o" ] && \
+	    egrep -q 'contrib/ntp/sntp/libevent/bufferevent_openssl.c' \
+	    ${OBJTOP}/usr.sbin/ntp/libntpevent/.depend.bufferevent_openssl.o ; then \
+		echo "Removing stale libevent dependencies"; \
+		rm -f ${OBJTOP}/usr.sbin/ntp/libntpevent/.depend.*; \
+	fi
 
 _worldtmp: .PHONY
 	@echo

usr.sbin/ntp/libntpevent/Makefile

@@ -1,6 +1,6 @@
 # $FreeBSD$
 
-.PATH: ${SRCTOP}/contrib/ntp/sntp/libevent
+.PATH: ${SRCTOP}/contrib/libevent
 
 LIB= ntpevent
 INTERNALLIB=
@@ -26,7 +26,7 @@ NTP_ATOMIC=noatomic
 .endif
 
 CFLAGS+= -I${SRCTOP}/contrib/ntp/include \
-	-I${SRCTOP}/contrib/ntp/sntp/libevent/include \
+	-I${SRCTOP}/contrib/libevent/include \
 	-I${.CURDIR}/
 
 CFLAGS+= -DHAVE_BSD_NICE -DHAVE_STDINT_H