doc isinf isnan function portability

[Kevin Ryde]

        * doc/autoconf.texi (Function Portability): Add notes on isinf and
        isnan as macros versus functions.

This arose in guile, where Andreas Vögele found AC_CHECK_FUNCS for
isinf and isnan insufficient on HP-UX.  We ended up with an
AC_LINK_IFELSE that included <math.h>, which is what I've tried to
alude to in the words below.  Not sure if that's as clear as it could
be.


@item @code{isinf}
@itemx @code{isnan}
@c @fuindex isinf
@c @fuindex isnan
@prindex @code{isinf}
@prindex @code{isnan}
The ISO C99 standard specifies that @code{isinf} and @code{isnan} are
macros.  On some systems just macros are available (eg.@: HP-UX), on
some systems both macros and functions (eg.@: glibc 2.3.2), but on some
systems only functions (eg.@: IRIX 6 and Solaris 8).

For maximum portability therefore a function test with
@code{AC_CHECK_FUNCS} or similar should be used as well as
@code{#ifdef}.  (@code{AC_CHECK_FUNCS} doesn't detect macros from
@code{<math.h>}.  A custom @code{AC_LINK_IFELSE} test could be made to
detect both though, if single @code{HAVE_ISINF} and @code{HAVE_ISNAN}
conditions are easier to use.)