Is this message correct in the current libcasper world order? Should it be an error?

Given the if block above will exit, no need for a else block here

casper cannot be null at this point given there checks above that exits

This will always be true there is a check above for that.

maybe if would be more readable to have a bunch of defined earlier:

#if HAVE_LIBCAPSER
#define tcrt_gethostbyname(domain) cpa_gethostbyname(capdns, domain)
#else 
#define tcrt_gethostbyname(domain) cpa_gethostbyname(capdns, domain)
#endif

with all functions like that

we compile with -DHAVE_LIBCASPER; does this #if work?

maybe lose the extra space here, like you do with HAVE_LIBCASPER stack variables in fns?

errx instead of perror/exit?

given that we already have cansandbox variable I'd prefer if we set if false if capsicum is not available, and drop the ENOSYS case

it seems odd to have both of these; afaik we need only src.opts.mk?

Not sure if I get this.

When we will have libcaspermock this will not be needed any more.

After one more look I get this ;)