This can be patched in saslutil.c like this:
/* gotta define gethostname ourselves on suns */
extern int gethostname(char *, int);
or by changing automake so it does not define HAVE_UNISTD_H for Solaris 11.
Solaris 11 has its own unistd.h - it might be nicer to use this rather than for dependencies to define their own versions of functions which were not previously available on Solaris < 11.
Alternatively we could submit a patch request to properly detect Solaris for this library.