Don't check for standard C89 headers

Don't check for

- ctype.h
- errno.h
- float.h
- limits.h
- math.h
- signal.h
- stdarg.h
- stdlib.h
- string.h
- time.h

Stop including non-standard headers

- malloc.h
- strings.h

debugXML.c

@@ -12,12 +12,8 @@
 #ifdef LIBXML_DEBUG_ENABLED
 
 #include <string.h>
-#ifdef HAVE_STDLIB_H
 #include <stdlib.h>
-#endif
-#ifdef HAVE_STRING_H
-#include <string.h>
-#endif
+
 #include <libxml/xmlmemory.h>
 #include <libxml/tree.h>
 #include <libxml/parser.h>