time64 build issues with -std=c99

 [2016/12/02 18:23:55.987] bson/time64.c: In function 'localtime64_r':

 [2016/12/02 18:23:55.987] bson/time64.c:680:9: warning: implicit declaration of function 'localtime_r' [-Wimplicit-function-declaration]

 [2016/12/02 18:23:55.987]          LOCALTIME_R(&safe_time, &safe_date);

 [2016/12/02 18:23:55.987]          ^

 [2016/12/02 18:23:55.988] bson/time64.c: In function 'localtime64':

 [2016/12/02 18:23:55.988] bson/time64.c:775:5: warning: implicit declaration of function 'tzset' [-Wimplicit-function-declaration]

 [2016/12/02 18:23:55.988]      tzset();

 [2016/12/02 18:23:55.988]      ^