C - Compilation of OpenSSL: No reference to BIO-functions

I'm trying to compile a c-program with openssl-references.
Starting position:

LinuxMint 17.1 and the development package "libssl-dev" is installed.

//...
#include <openssl/bio.h>
#include <openssl/err.h>
#include <openssl/ssl.h>
//...
void send_smtp_request(BIO *bio, const char *req){
       BIO_puts(bio, req);
       BIO_flush(bio);
       printf("%s", req);
}
//... more code

If I compile the code with:
gcc -o client bio-ssl-smtpcli2.c

I get the this error:
/tmp/ccCHrti2.o: In function 'send_smtp_request':
bio-ssl-smtpcli2.c:(.text+0x1f): undefined reference to 'BIO_puts'
bio-ssl-smtpcli2.c:(.text+0x3a): undefined reference to 'BIO_ctrl'

Someone has an idea how to fix this?