Use full path for stub specification

Previously the test stub specification used a Tempfile.  On travis-ci
this would result in the tests attempting to write to /gems instead of
into the test-specific temp directory.

Now the stub specification is written into the test-specific temp
directory which, I hope, fixes the travis failure.
Do not clean for cmake

Makefiles created with cmake do not seem to have a general "clean"
target.  This may mean that this commit for #596 breaks cmake support,
but I don't know of a cmake project to test it with.