knet as optional dependency because knet does not work with musl

[jirib]

As libknet1 does not work with musl and corosync can't be told via configure script to disable knet. It seems build cannot pass configure phase on musl distros (eg. Void Linux, Alpine Linux) where one still could use 'udpu' or 'udp'.

checking for libknet... no
configure: error: Package requirements (libknet) were not met:
No package 'libknet' found
Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Or if anybody would like to help to make kronosnet build on musl distros, it would be even better.

See kronosnet/kronosnet#249 (comment)

[jfriesse]

@jirib
Thank you for the report! I'm really not keen to allow corosync compile without knet. UDP/UDPU are mostly deprecated, because these transports doesn't support multiple links, encrypted traffic, dynamic path MTU, ...

Said all above, I believe correct way is to make knet compilable with musl, ether by fixing knet or musl. And because knet works (at least) on FreeBSD (so different OS with different syscalls, different compiler (clang) and completely different libc) it shouldn't be that big deal.

Hope you don't mind if I close this issue and will focus on knet one.