[GH #892] Implement our own capture, rm lib/IO/CaptureOutput.pm

No need to ship a IO::CaptureOutput.
Use the new capture function in t/configure/033-step.t and
t/configure/034-step.t. t/configure/034-step.t tests now the
new replace_stash option correctly. See [GH #891].
Merge branch 'smoke-me/test-capture-gh892'
[cygwin] disable powl in string api

the probe seems to work okay, but it doesn't.

need math.h for all platforms in string/api.c.
[test] disable signal tests again

hang when being run in parallel -j4