Time zone conversion C API on Linux, anyone?

I'm looking for something that I presumed would be very simple - given local Unix time in a specific time zone (specified as a string, e.g., "America/New_York" - note that's not my local time), get the corresponding time value in GMT. I.e., something along the lines of

time_t get_gmt_time(time_t local_time,
                    const char* time_zone);

As deceptively simple as it sounds, the closest I could find was the following code snippet from timegm's man page:

       #include <time.h>
       #include <stdlib.h>

       time_t
       my_timegm(struct tm *tm)
       {
           time_t ret;
           char *tz;

           tz = getenv("TZ");
           setenv("TZ", "", 1);
           tzset();
           ret = mktime(tm);
           if (tz)
               setenv("TZ", tz, 1);
           else
               unsetenv("TZ");
           tzset();
           return ret;
       } 

There gotta be a better way than this belligerently not thread-safe abomination, right? Right??