Fix spurious dependency bumps, bring back to 5.6

Note - while I tested things extensively, it is not easy to test 5.6 currently,
as a lot of the depchain was subject to bitrot. I currently have pending fixes
for File::Spec, Capture::Tiny, File::Temp and File::Path, each containing some
trivial nit preventing otherwise 5.6-compliant code from working

Patches will be sent upstream in due time
invoke methods, not functions (allow overriding)
introduce check_prog_pkg_config
fix warnings and invocation issues at check_prog
improve tests for progs

* in times of pre-built binaries, $Config{} doesn't tell anything but
  the build machine had access to that
  For machine stuff (word-width) relying on it is ok, for runtime stuff
  (tools, et. al) relying on it is dangerous (even if I never seen a
  Unix without grep - what's on Haiku, VMS, ...?)
* add test for pkg-config
bump version to 0.23