cmake: epee: use var from FindOpenSSL.cmake

This fixes linking when path to openssl
is defined manually:
cmake -DOPENSSL_ROOT_DIR='/usr/include/openssl-1.0;/usr/lib/openssl-1.0' ...

This is useful for building with OpenSSL v1.0
when default system installation is v1.1.

The linking error is undefined SSL_load_error_strings symbol.
This is due to -L /usr/lib/openssl-1.0 not making it onto
the linkline (so -lssl pulls in the default system openssl).

contrib/epee/src/CMakeLists.txt

@@ -49,6 +49,5 @@ target_link_libraries(epee
     easylogging
     ${Boost_FILESYSTEM_LIBRARY}
   PRIVATE
-    ssl
-    crypto
+    ${OPENSSL_LIBRARIES}
     ${EXTRA_LIBRARIES})