clang -O2  -Wall -Wextra -Werror=implicit-function-declaration unistd/setregid.c -o unistd/setregid -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm  -lintl
unistd/setregid.c:12:7: error: implicit declaration of function 'setregid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        if ( setregid(rgid, egid) < 0 )
             ^
1 error generated.