cmake: check openssl version

See #835 for memory leak issues with OpenSSL >= 1.1

build/CMakeLists.txt

@@ -322,6 +322,10 @@ endif()
 find_package ( OpenSSL REQUIRED )
 if(NOT DEFINED OPENSSL_INCLUDE_DIR)
   message(SEND_ERROR "Could not find OpenSSL. Please download and install it first!")
+else()
+  if(NOT (OPENSSL_VERSION VERSION_LESS 1.1))
+    message(SEND_ERROR "Unsupported OpenSSL version: ${OPENSSL_VERSION} (required < 1.1)")
+  endif()
 endif()
 
 if (WITH_UPNP)