tests: some tests make no sense if "$CC -c -o" doesn't work

tests: some tests make no sense if "$CC -c -o" doesn't work

So just skip them, to avoid spurious failures when running
"make check-no-cc-c-o".

* t/ax/am-test-lib.sh (require_tool): New requirement '-c-o'.
* t/subobj10.sh ($required): Add it.
* gen-testsuite-part (%depmodes): Adjust so that tests that
use 'makedepend' will be skipped if the compiler is being
forced not to grasp "-c -o".

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