This is a C api. To use it you need to include (at least) openssl/ssl.h and to link your program with libssl library.
How to get libssl API to compile with it
on Debian base ( debian, ubuntu, ... ) you would need libssl-dev : apt-get install libssl-dev.
on Redhat base ( RedHat, Fedora, ... ) you would need openssl-devel : yum install openssl-devel
You can get sources directly too to compile statically over it.