…4 support (refs avrdudes#1782)

This ancient libgpiod version is still shipped on Ubuntu 20.04 LTS.
Add support for it by adding another helper.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

Commit 8390909 introduced a regression with
libgpiod versions below 2.0.

Fix it by introducing a tiny helper function which returns
the GPIO number (offset).

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

The standard cmake messages of pkg_check_modules might be confusing
when being used as here, i.e. when searching for the highest compatible
version.

So suppress these standard messages and print manual ones instead.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

Due to a copy and paste error, the required define for
using libgpido at all was not set and thus the library
was not used at all.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

We cannot print the exact version we detected, but we
can give a hint about the "version class" at least.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>