build: check for gmtime_r and localtime_r with the new macro.

This makes sure that the two functions are found when building on mingw32,
as the functions are defined as static inline rather than as library
symbols.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>