Only I figured out that I need to have that command affect two different sections of code, not one like I originally asked about.
I need to enclose each of these
#define HAVE_GMTIME_R 1
#define HAVE_LOCALTIME_R 1
in opening and closing braces so they look like this:
/* #define HAVE_GMTIME_R 1 */
/* #define HAVE_LOCALTIME_R 1 */
Thanks for your help iainr!