Fix malformed if() check when collada is missing

The line
  `if( ${COLLADA_DOM_VERSION} VERSION_LESS "2.5.0" )`
would evaluate to
  `if( VERSION_LESS "2.5.0" )`
and throw an 'Unknown arguments specified' CMake error (tested: CMake v3.9.2)
if collada is not found. Also avoid showing a misleading status message.

CMakeLists.txt

@@ -600,12 +600,14 @@ if( OPT_COLLADA )
   #find_package(COLLADA_DOM 2.4 COMPONENTS 1.5 1.4 PATHS ${COLLADA_PATH})
   find_package(COLLADA_DOM 2.3 COMPONENTS 1.5 PATHS ${COLLADA_PATH})
 
-  if( ${COLLADA_DOM_VERSION} VERSION_LESS "2.5.0" )
-    message(STATUS "collada version ${COLLADA_DOM_VERSION} does not support write to memory")
-    set(COLLADA_SUPPORT_WRITE_MEMORY 0)
-  else()
-    message(STATUS "collada version ${COLLADA_DOM_VERSION} supports write to memory")
-    set(COLLADA_SUPPORT_WRITE_MEMORY 1)
+  if( COLLADA_DOM_FOUND )
+    if( COLLADA_DOM_VERSION VERSION_LESS "2.5.0" )
+      message(STATUS "collada version ${COLLADA_DOM_VERSION} does not support write to memory")
+      set(COLLADA_SUPPORT_WRITE_MEMORY 0)
+    else()
+      message(STATUS "collada version ${COLLADA_DOM_VERSION} supports write to memory")
+      set(COLLADA_SUPPORT_WRITE_MEMORY 1)
+    endif()
   endif()
 
   find_package(ZLIB)