HAVE_ALLOCA_H is missing

[Ayamura KIKUCHI]

Files "config.h.in" and "configure.in" in rsync cvs repository should
be changed to detect whether a system has <alloca.h> or not.

rsync_cvs/popt/system.h:

#ifndef __GNUC__
# if HAVE_ALLOCA_H
#  include <alloca.h>
# else
#  ifdef _AIX
#pragma alloca
#  else
#   ifndef alloca /* predefined by HP cc +Olibcalls */
char *alloca ();
#   endif
#  endif
# endif
#elif defined(__GNUC__) && defined(__STRICT_ANSI__)
#define alloca __builtin_alloca
#endif

-- ayamura