Unbreak dlopen detection on FreeBSD

Previous debian-sourced patch was broken, it set DL_LIBS="-ldl" even if
the dl library was not used. On FreeBSD it does not exist, resulting in
failed build if ZERO was enabled.
Also fixed two other similar issues, even if they worked on FreeBSD.

configure.ac

@@ -1643,20 +1643,22 @@ AC_TYPE_SIGNAL
 AC_CHECK_FUNCS([accept daemon getaddrinfo freeaddrinfo gethostbyname2_r gethostbyaddr_r gethostbyname_r getservbyname_r getopt getopt_long gmtime_r gettimeofday localtime_r inet_ntop inet_pton memchr memset select socket strcasecmp strchr strdup strncasecmp strtol strtoul twalk tsearch tfind tdelete tdestroy vasprintf strsep vsprintf vsnprintf writev getline])
 
 save_LIBS="$LIBS"
+LIBS=""
 DL_LIBS=""
 if test "${ENABLE_ZERO}" = "true" ; then
 	AC_SEARCH_LIBS(dlopen, dl,
-		DL_LIBS="-ldl"
 		AC_DEFINE(HAVE_DLOPEN, 1, [Define if you have dlopen]))
+	DL_LIBS="$LIBS"
 fi
 AC_SUBST([DL_LIBS])
 LIBS="$save_LIBS"
 
 save_LIBS="$LIBS"
 M_LIBS=""
+LIBS=""
 AC_SEARCH_LIBS(lrint, m,
-       M_LIBS="-lm"
        AC_DEFINE(HAVE_LRINT, 1, [Define if you have lrint]))
+M_LIBS="$LIBS"
 AC_SUBST([M_LIBS])
 LIBS="$save_LIBS"
 
@@ -1681,8 +1683,9 @@ AS_IF([test "x$with_systemdsystemunitdir" != "xno"],
 AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])
 save_LIBS="$LIBS"
 MQ_LIBS=""
-AC_SEARCH_LIBS([mq_getattr], [rt],
-		[MQ_LIBS="-lrt"])
+LIBS=""
+AC_SEARCH_LIBS([mq_getattr], [rt])
+MQ_LIBS="$LIBS"
 AC_SUBST([MQ_LIBS])
 LIBS="$save_LIBS"