[krbdev.mit.edu #7526] "Invalid argument" error for nonexistent KDC hostname

[Greg Hudson via RT]

Nalin suggested using AC_USE_SYSTEM_EXTENSIONS, which tries to define 
the "use all system extensions" flags on various systems (_GNU_SOURCE 
with glibc, __EXTENSIONS__ and _POSIX_PTHREAD_SEMANTICS on Solaris, 
etc.).

Tom is wary that defining symbols like this in a library might make it 
incompatible with an application which doesn't use those symbols.  The 
potential exists (for instance, https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40411 and its referents 
document issues which can arise from using the native "-xc99=all" on 
Solaris), but we don't have any specific knowledge of such an issue 
arising just from defining symbols.

However, _GNU_SOURCE does have at least one detrimental effect: it 
causes the Gnu version of strerror_r to be used instead of the default 
POSIX version, which would require us to conditionalize every place we 
use that function.  (The standard version returns an int and always puts 
the result in the caller buffer; the Gnu version returns a char * which 
may point to the buffer or may point somewhere else.)