CMake: Show a helpful error message with CMake 2.8+ if Boost has not been found.

git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@819 67173fc5-114c-0410-ac8e-9d2fd5bffc1f

CMakeLists.txt

@@ -32,6 +32,7 @@ IF ( ENABLE_BOOST_WORKAROUND )
 	ADD_DEFINITIONS( -DASSIMP_BUILD_BOOST_WORKAROUND )
 	MESSAGE( STATUS "Building a non-boost version of Assimp." )
 ELSE ( ENABLE_BOOST_WORKAROUND )
+	SET( Boost_DETAILED_FAILURE_MSG ON )
 	FIND_PACKAGE( Boost 1.35 )
 
 	IF ( NOT Boost_FOUND )