headers: make check of system header explicit

headers: make check of system header explicit

Exploiting knowledge of internal behavior of other macros can
be dangerous, especially since AC_CHECK_HEADERS_ONCE is designed
to be cheap on subsequent use.

* m4/netdb_h.m4 (gl_HEADER_NETDB): Don't exploit knowledge of
gl_CHECK_NEXT_HEADER internals, but call AC_CHECK_HEADERS_ONCE
ourselves.
* m4/search_h.m4 (gl_SEARCH_H): Likewise.
* m4/sys_select_h.m4 (gl_HEADER_SYS_SELECT): Likewise.
* m4/sys_time_h.m4 (gl_HEADER_SYS_TIME_H_BODY): Likewise.
* m4/inttypes.m4 (gl_INTTYPES_H): Likewise, for gt_INTTYPES_PRI
internals.
* m4/wchar.m4 (gl_WCHAR_H): Skip followup test if header is
missing.
Suggested by Bruno Haible.

Signed-off-by: Eric Blake <ebb9@byu.net>