x11/mate-panel: fix build with clang 15

During an exp-run for llvm 15 (see bug 265425), it turned out that
x11/mate-panel failed to build with clang 15:

  clock-location.c:454:22: error: incompatible pointer to integer conversion assigning to 'glong' (aka 'long') from 'char *(int, int)' [-Wint-conversion]
          sys_timezone = timezone;
                       ^ ~~~~~~~~
  clock-location.c:462:24: error: incompatible pointer to integer conversion assigning to 'glong' (aka 'long') from 'char *(int, int)' [-Wint-conversion]
          local_timezone = timezone;
                         ^ ~~~~~~~~

This is because 'timezone' as a global external variable does not exist
on FreeBSD: it is a glibc-ism. Use struct tm's tm_gmtoff field instead,
which has its sign reversed from the glibc global. There is also no need
to manually account for DST, as tm_gmtoff includes that.

PR:		268795
Approved by:	portmgr (tcberner)
MFH:		2023Q1

x11/mate-panel/files/patch-applets_clock_clock-location.c

@@ -0,0 +1,30 @@
+--- applets/clock/clock-location.c.orig	2022-01-26 15:27:16 UTC
++++ applets/clock/clock-location.c
+@@ -451,19 +451,27 @@ clock_location_get_offset (ClockLocation *loc)
+
+         unsetenv ("TZ");
+         tm = localtime (&t);
++#ifdef __FreeBSD__
++        sys_timezone = -tm->tm_gmtoff;
++#else        
+         sys_timezone = timezone;
+
+         if (tm->tm_isdst > 0) {
+                 sys_timezone -= 3600;
+         }
++#endif
+
+         setenv ("TZ", priv->timezone, 1);
+         tm = localtime (&t);
++#ifdef __FreeBSD__
++        local_timezone = -tm->tm_gmtoff;
++#else        
+         local_timezone = timezone;
+
+         if (tm->tm_isdst > 0) {
+                 local_timezone -= 3600;
+         }
++#endif
+
+         offset = local_timezone - sys_timezone;
+