clang -O2  -Wall -Wextra -Werror=implicit-function-declaration unistd/getpgid.c -o unistd/getpgid -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm  -lintl
unistd/getpgid.c:9:7: error: implicit declaration of function 'getpgid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        if ( getpgid(0) == (pid_t) -1 )
             ^
1 error generated.