Openvsp uses libxml, without explicitly looking for it; in CMake 3.9
this was still OK as some other module included it. That implicit
search has gone away, so add it explicitly where it is used.

--- src/vsp/CMakeLists.txt.orig	2017-09-16 22:18:44 UTC
+++ src/vsp/CMakeLists.txt
@@ -3,6 +3,8 @@ CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
 # include the CTest framework
 include(CTest)
 
+find_package(libXml2)
+
 IF("${CMAKE_SYSTEM}" MATCHES "Linux")
 	SET( LINUX_LIBS
 		pthread