dealing with built-in functions

[Brian Dessent]

flo@redflo.de wrote:

> Autoconf did not help me here automatically. But maybe i have to change
> something in configure.ac?
> So what is the "right way"? Use -D_GNU_SOURCE while compiling or add a
> #define? I decided to add the #define, and it works. Thank you for your
> explanation and help!

It looks like you can just add the AC_USE_SYSTEM_EXTENSIONS macro
somewhere early in your configure.ac to get a combined set of related
checks. 
<http://www.gnu.org/software/autoconf/manual/html_node/Posix-Variants.html>.

Alternatively, you could call AC_PROG_CC_C99 which would end up setting
CC to gcc -std=gnu99, which in turn should cause _ISOC99_SOURCE to be
defined.  This may be a bigger hammer than you require, depending on how
much C99 your code requires. 
<http://www.gnu.org/software/autoconf/manual/html_node/C-Compiler.html#index-AC_005fPROG_005fCC_005fC99-750>

Brian