OpenSSL appears to be installed on OS X but no headers found by GCC

I'm trying to use the MD5 part of OpenSSL but I cannot get the program to compile because GCC is complaining that it can't find the OpenSSL headers. I find this strange because OpenSSL appears to be already installed on OSX.

% openssl version LibreSSL 2.8.3

GCC spits out this

% gcc -lcrypto md5.c   ld: library not found for -lcryptoclang: error: linker command failed with exit code 1 (use -v to see invocation)

Are there headers on OS X or do I need to install OpenSSL again? This is very confusing because the library already appears to be installed by default.