On x86 Linux at least, alloca is not in libc, it's strictly a compiler builtin function. You test for with with the AC_CHECK_FUNCS macro which does its work by trying to use the address of the function. I've tried a number of different versions and they all seem to decide that you can't want the builtin function because you're using an address so they emit a normal external symbol. Of course, this fails with a linker error. I think you need a different test if you're using gcc.
Created attachment 10000 [details] [PATCH] Use AC_FUNC_ALLOCA, which handles this issue
make the patch findable
Fixed on trunk, but I won't backport it; the only place this matters is in the poll code and most places you have alloca support you probably have VLA support too. Thanks for the patch, Noah. http://svn.apache.org/viewcvs?rev=239697&view=rev