Autoconf Reference Guide - 5.1.2 Default Includes

Several tests depend upon a set of header files. Since these headers are not universally available, tests actually have to provide a set of protected includes, such as:

Unless you know exactly what you are doing, you should avoid using unconditional includes, and check the existence of the headers you include beforehand (see Header Files).

Most generic macros use the following macro to provide the default set of includes:

— Macro: AC_INCLUDES_DEFAULT ([include-directives])

Expand to include-directives if defined, otherwise to:
          #include <stdio.h>
          #if HAVE_SYS_TYPES_H
          # include <sys/types.h>
          #endif
          #if HAVE_SYS_STAT_H
          # include <sys/stat.h>
          #endif
          #if STDC_HEADERS
          # include <stdlib.h>
          # include <stddef.h>
          #else
          # if HAVE_STDLIB_H
          #  include <stdlib.h>
          # endif
          #endif
          #if HAVE_STRING_H
          # if !STDC_HEADERS && HAVE_MEMORY_H
          #  include <memory.h>
          # endif
          # include <string.h>
          #endif
          #if HAVE_STRINGS_H
          # include <strings.h>
          #endif
          #if HAVE_INTTYPES_H
          # include <inttypes.h>
          #endif
          #if HAVE_STDINT_H
          # include <stdint.h>
          #endif
          #if HAVE_UNISTD_H
          # include <unistd.h>
          #endif
     
If the default includes are used, then check for the presence of these headers and their compatibility, i.e., you don't need to run AC_HEADER_STDC, nor check for stdlib.h etc.
These headers are checked for in the same order as they are included. For instance, on some systems string.h and strings.h both exist, but conflict. Then HAVE_STRING_H is defined, not HAVE_STRINGS_H.