utils.c: pathconf call, check for _PC_PATH_MAX instead of HAVE_UNISTD_H.

utils.c: pathconf call, check for _PC_PATH_MAX instead of HAVE_UNISTD_H.

This check in utils.c for HAVE_UNISTD_H is being used as proxy for
"HAVE_PATHCONF", as pathconf is supposed to be declared in unistd.h.

It's possible that there are systems out there that have realpath,
unistd.h and alloca, but not pathconf+_PC_PATH_MAX.  I don't know of
any by heart, but if we import gnulib's unistd module (which a
following patch will do), then unistd.h ends up always available, so
the check ends up incorrect.  As pathconf is being called with
_PC_PATH_MAX, check for that instead.

gdb/
2013-07-01  Pedro Alves  <palves@redhat.com>

	* utils.c <pathconf/_PC_PATH_MAX use>: Check if _PC_PATH_MAX is
	defined instead of checking HAVE_UNISTD_H.