Add string.h if necessary to define strlen, etc.

Add string.h if necessary to define strlen, etc.
Add stdlib.h if present & necessary for malloc definition. (otherwise
	define ourselves) - based on other code present.
Change ifdef POSIX to ifdef HAS_STDLIB_H
Check for stdlib.h in configure.in

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@5757 dc483132-0cff-0310-8789-dd5450dbe970