Support CMake versions <= 3.8.

CMakeLists.txt

@@ -1,8 +1,8 @@
 cmake_minimum_required(VERSION 3.7)
 project(libdatachannel
-	DESCRIPTION "WebRTC Data Channels Library"
 	VERSION 0.9.4
 	LANGUAGES CXX)
+set(DESCRIPTION "WebRTC Data Channels Library")
 
 # Options
 option(USE_GNUTLS "Use GnuTLS instead of OpenSSL" OFF)