Add system libraries when checking OpenSSL functions.

If OpenSSL is a static library, linking the test programs will require
system libraries.

CMakeLists.txt

@@ -335,7 +335,7 @@ check_include_file(openssl/rand.h HAVE_OPENSSL_RAND_H)
 check_include_file(openssl/conf.h HAVE_OPENSSL_CONF_H)
 check_include_file(openssl/engine.h HAVE_OPENSSL_ENGINE_H)
 
-set(CMAKE_REQUIRED_LIBRARIES ${OPENSSL_LIBRARIES})
+set(CMAKE_REQUIRED_LIBRARIES ${OPENSSL_LIBRARIES} ${getdns_system_libs})
 check_function_exists(DSA_SIG_set0 HAVE_DSA_SIG_SET0)
 check_function_exists(DSA_set0_pqg HAVE_DSA_SET0_PQG)
 check_function_exists(DSA_set0_key HAVE_DSA_SET0_KEY)