Since commit v3.3.0-rc1~62^2~5 (cmTarget: Store only cmListFileContext
for CMP0023 handling, 2015-05-18) a call to target_link_libraries on a
target that was defined in another (non-ancestor) directory crashes
because no execution context is left active.  Fix this by getting the
execution context from the actual cmMakefile where the current
target_link_libraries call takes place.  Test this by verifying that
such calls correctly produce an error diagnostic instead of crashing.