Work around missing UseCGAL.cmake

CGAL 6 no longer provides UseCGAL.cmake.
We probably don't need it (we use other variables
which are set by the CGAL's CMake config) so
just skip it if not provided. Closes #1100.

modules/cgal/Setup.cmake

@@ -5,7 +5,9 @@ file(WRITE "${CMAKE_BINARY_DIR}/build_info/CGAL" "ok=True")
 set(CGAL_DONT_OVERRIDE_CMAKE_FLAGS TRUE CACHE BOOL "Don't override flags")
 # Ignore CGAL_CXX_FLAGS from previous cmake run, if any
 set( CGAL_CXX_FLAGS "" CACHE INTERNAL "" FORCE)
-include(${CGAL_USE_FILE})
+if (DEFINED CGAL_USE_FILE)
+  include(${CGAL_USE_FILE})
+endif()
 # Set "standard" include/link variables from CGAL's
 set(CGAL_INCLUDE_PATH ${CGAL_INCLUDE_DIRS} CACHE INTERNAL "" FORCE)
 set(CGAL_LINK_PATH ${CGAL_LIBRARIES_DIR} CACHE INTERNAL "" FORCE)