To compile it you'll need to tell it to link to libcurl, e.g. with -lcurl on gcc:
curl https://ifconfig.me --libcurl ip_fetcher.c
# Output: your ip address, and a file ip_fetcher.c
gcc -o ip_fetcher ip_fetcher.c -lcurl
# Output: no errors, just a file ip_fetcher
./ip_fetcher
# Output: your ip address
(I'm sure most people are saying "no duh" right now, but I'm probably not the only one on here who doesn't write C code every day!)