ipaccess: require tcp keepalive related defines
ipaccess: require tcp keepalive related defines

Fail the compilation if TCP_KEEPIDLE, TCP_KEEPINTVL, TCP_KEEPCNT or
TCP_USER_TIMEOUT are not defined.

Harald wrote:
> What we want to prevent is the user configuring timeouts, assuming
> they would be installed into the kernel TCP stack, which then simply
> end up no-ops becaus somehow the libc didn't define them or the right
> #include file was not present at compile time.
>
> [...] Apparently TCP_KEEP{IDLE,INTVL,CNT} were introduced with kernel
> 2.4 and TCP_USER_TIMEOUT with 2.6.37. I think it's fair to say that
> using a modern/master libosmo-* on such old systems might fail for
> various other reasons (eventfd, ...) and cannot be considered a valid
> configuration anyway.

Closes: OS#5786
Change-Id: Idc0ff1ff02ce4b994692d8213c14c0b2caad756e

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis