Add check for FRESH_SOURCE_DIR in Callergraph CMake

Add an if block at the beginning of the CmakeLists.txt in the Utilities/Dox
directory that checks if the TEST_VISTA_FRESH_SOURCE_DIR is defined.

If it is: the code should run as normal to create the tests.  If it is not
defined, skip the code and print a warning that the variable will need to
be defined before the tests can be created.

Change-Id: I50996dd8f2107e56e191093697852e2d99af130b