check-cc-no-c-o: avoid a spurious failure

check-cc-no-c-o: avoid a spurious failure

* t/am-prog-cc-c-o.sh: In this test, by relying on the knowledge
that we are running under the aegis of the 'check-cc-no-c-o'
maintainer-specific target, knowledge given us by ...
* t/Makefile.in (check-cc-no-c-o) : ... the new environment
variable 'AM_TESTSUITE_SIMULATING_NO_CC_C_O', set to a value
of "yes" by this rule.
* t/ax/test-defs.in: Initialize the new variable to "no" by
default, and add an explanatory comment.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>