Portability fixes

[0-wiz-0]

These patches are from pkgsrc and increase portability of dunst to other operating systems.

[bynect]

Related to #1171.

For the features.h it seems like it is used just to check __GLIBC__ once, so maybe we can change it so we never need it?

Similarly for the button codes, I think we can remove the reliance on linux codes and just define an enum. (since they are arbitrary number anyway and are not used nowhere else)
What do you think?@fwsmit

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.46%. Comparing base (ab38321) to head (8baccb8).

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1288   +/-   ##
=======================================
  Coverage   64.46%   64.46%           
=======================================
  Files          48       48           
  Lines        7920     7920           
=======================================
  Hits         5106     5106           
  Misses       2814     2814           

Flag	Coverage Δ
unittests	64.46% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[fwsmit]

What operating systems are you trying to port to? That would be good to know.
The button codes are plain numbers, but we get them from wayland / X11 I believe. So we need to use those specific numbers. I didn't check it though

[0-wiz-0]

pkgsrc is cross-platform.
The evdev patches are needed at least on NetBSD, and the find patch is (I think) needed on Solaris/Illumos.

[bynect]

I resolved a conflict with #1290.
Also thanks for the pr, I'll merge it 👍🏻