Use system extensions conditionally?

[Fredrik Tolf]

Dear list,

I'm trying to write a program that can use custom stdio streams on both 
Linux/glibc and FreeBSD, and to that end, I'm trying to modify to autoconf 
script to properly detect these and turn on the necessary compiler flags 
to support them.

On glibc, I need to define `_GNU_SOURCE' in order to gain access to 
fopencookie() and cookie_io_functions_t. Autoconf neatly supports turning 
this on with `AC_USE_SYSTEM_EXTENSIONS(_GNU_SOURCE)', but I can't quite 
seem to figure out how to only do this when necessary. Currently, I'm 
trying to do it like this:

    HAS_FOPENCOOKIE=yes
    AC_CHECK_FUNC(fopencookie, [AC_USE_SYSTEM_EXTENSIONS(_GNU_SOURCE)], 
[HAS_FOPENCOOKIE=no])
    AC_CHECK_MEMBER([cookie_io_functions_t.read], [], [HAS_FOPENCOOKIE=no])

This /works/, per se, but autoconf complains loudly when I try to generate 
the configure script, several times, that `AC_COMPILE_IFELSE was called 
before AC_USE_SYSTEM_EXTENSIONS'. Apparently, it thinks it is really bad 
practice to actually do any tests before turning on system extensions.

What should I do about this? Just ignore the warnings? Do the necessary 
AC_DEFINE manually instead? (This seems ugly, since I'd need to define an 
`AH_TEMPLATE` as well and all that.) Just turn on all extensions that I 
may or may not use, unconditionally at the top of the file? Something 
completely different?

Thanks for reading!

--
Fredrik Tolf