Macos – GCC Warning: implicit declaration of function ‘puts’ is invalid in C99

I am beginning Zed Shaw's Learn C The Hard Way. I have downloaded XCode and the Command Line Tools. But when I compile the very first program:

int main(int argc, char *argv[]) {
     puts("Hello world."); 
     return 0;
 }

I get this warning:

ex1.c:2:1: warning: implicit declaration of function 'puts' is invalid
in C99
[-Wimplicit-function-declaration]

The program does compile and execute correctly.

I'm using OSX 10.8.3. Entering 'gcc -v' gives:

Using built-in specs. Target: i686-apple-darwin11 Configured with:
/private/var/tmp/llvmgcc42/llvmgcc42-2336.11~182/src/configure
–disable-checking –enable-werror –prefix=/Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2 –mandir=/share/man –enable-languages=c,objc,c++,obj-c++ –program-prefix=llvm- –program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ –with-slibdir=/usr/lib –build=i686-apple-darwin11 –enable-llvm=/private/var/tmp/llvmgcc42/llvmgcc42-2336.11~182/dst-llvmCore/Developer/usr/local
–program-prefix=i686-apple-darwin11- –host=x86_64-apple-darwin11 –target=i686-apple-darwin11 –with-gxx-include-dir=/usr/include/c++/4.2.1 Thread model: posix gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)

Please help.