autoconf-2.57: unclear documentation for AC_CHECK_DECL and AC_CHECK_DECLS

[Nelson H. F. Beebe]

The autoconf-2.57 manual's documentation for AC_CHECK_DECL and
AC_CHECK_DECLS is unclear.  It reads:

>> ...
>>  - Macro: AC_CHECK_DECL (SYMBOL, [ACTION-IF-FOUND],
>>           [ACTION-IF-NOT-FOUND], [INCLUDES = `default-includes'])
>>      If SYMBOL (a function or a variable) is not declared in INCLUDES
>>      and a declaration is needed, run the shell commands
>>      ACTION-IF-NOT-FOUND, otherwise ACTION-IF-FOUND.  If no INCLUDES
>>      are specified, the default includes are used (*note Default
>>      Includes::).
>> ...

There are no examples in the manual of the specification of INCLUDES,
nor could I spot any in the installed autoconf libraries.  

Does it want a list of header files: [stdio.h stddef.h math.h]?

Or does it want include statements: [
#include <stdio.h>
#include <stddef.h>
#include <math.h>
]?

Or does it want a key=value assignment: INCLUDES=[...]?

By experiment, I found that this works:

        MATH_INCLUDES="
        #include <math.h>
        #if defined(HAVE_SUNMATH_H)
        #include <sunmath.h>
        #endif
        "
        AC_CHECK_DECLS([ilogbf, irintf, nintf],,,[$MATH_INCLUDES])

That is, AC_CHECK_DECLS expects its fourth argument to be a block of C
code that includes files, and possibly does other things as well.
No INCLUDES= keyword expected.

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