cc -pthread  -Wall -Wextra -Werror=implicit-function-declaration ctype/toupper_l.c -o ctype/toupper_l -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm -lpthread  -liconv
ctype/toupper_l.c:15:12: error: call to undeclared function 'toupper_l'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   15 |         char c3 = toupper_l(c1, locale);
      |                   ^
ctype/toupper_l.c:15:12: note: did you mean 'toupper'?
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_ctype.h:303:1: note: 'toupper' declared here
  303 | toupper(int _c)
      | ^
1 error generated.