Some fixes to compat code for legacy OS

[barracuda156]

This is not a complete solution yet. At the very minimum, we need open_memstream

/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_net_openiked/openiked/work/openiked-portable-7.4/iked/ikev2.c: In function 'ikev2_handle_delete':
/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_net_openiked/openiked/work/openiked-portable-7.4/iked/ikev2.c:3099:21: error: implicit declaration of function 'open_memstream' [-Wimplicit-function-declaration]
 3099 |         if ((spif = open_memstream(&spibuf, &dummy)) == NULL) {
      |                     ^~~~~~~~~~~~~~

Fails at 91%, so possibly this is the last one.

@tobhe I do not know what to do with other systems. I am neither particularly motivated to deal them, nor can test anything. But maybe restore headers at least?

[tobhe]

@tobhe I do not know what to do with other systems. I am neither particularly motivated to deal them, nor can test anything. But maybe restore headers at least?

I think it is fine to ignore them for now until someone cares enough to report an issue. Newer version of posix standardized arc4random and getentropy so only older releases should be affected anyway.

[barracuda156]

@tobhe This builds for me without legacy-support now.