alloca redefinition on freebsd

[Vincent Torri]

Hey,

if one uses AC_FUNC_ALLOCA, the autoconf documentation gives the way to 
correctly define alloca.

On FreeBSD, alloca is already defined in stdlib.h. So if stdlib.h is 
included before that piece of code, a warning is displayed by gcc.

Shouldn't a guard be added in the piece of code of the docmentation of 
AC_FUNC_ALLOCA in order to avoid that warning (like #ifndef alloca etc...) 
?

thank you

Vincent Torri