* pty-int.h: Add getutmpx() prototype if needed

	* pty-int.h: Add getutmpx() prototype if needed

	* configure.in: If getutmpx() exists on the system, test if a
	prototype is provided by the system headers.

This was picked up by gcc -Wall. Under Linux glibc 2.1, the system
header file <utmpx.h> provides a prototype for getutmpx() only if
_GNU_SOURCE is defined (GNU extensions). The test offeres a prototype
(which matches in this case) - if the prototype conflicts with a system
provided one, we do not provide it.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@12477 dc483132-0cff-0310-8789-dd5450dbe970