(autoconf.info)Generic Headers

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4.4.2 Generic Header Checks
---------------------------

These macros are used to find system header files not covered by the
particular test macros.  If you need to check the contents of a header
as well as find out whether it is present, you have to write your own
test for it (Note: Writing Tests).

 -- Macro: AC_CHECK_HEADER (HEADER-FILE, [ACTION-IF-FOUND [,
          ACTION-IF-NOT-FOUND]])
     If the system header file HEADER-FILE exists, execute shell
     commands ACTION-IF-FOUND, otherwise execute ACTION-IF-NOT-FOUND.
     If you just want to define a symbol if the header file is
     available, consider using 'AC_CHECK_HEADERS' instead.

 -- Macro: AC_CHECK_HEADERS (HEADER-FILE... [, ACTION-IF-FOUND [,
          ACTION-IF-NOT-FOUND]])
     For each given system header file HEADER-FILE in the
     whitespace-separated argument list that exists, define
     'HAVE_HEADER-FILE' (in all capitals).  If ACTION-IF-FOUND is given,
     it is additional shell code to execute when one of the header files
     is found.  You can give it a value of 'break' to break out of the
     loop on the first match.  If ACTION-IF-NOT-FOUND is given, it is
     executed when one of the header files is not found.