bug#27346: module tests fail to compile with gcc 4.8.5

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Mon, 12 Jun 2017 13:37:18 -0400
> 
> Current master on rhel7.3.
> gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11)
> 
> After building with-modules:
>     cd test/data/emacs-module
>     make
>     
>     gcc -std=gnu11 -I../../../src -fPIC -g3 -O2 -c -o mod-test.o mod-test.c
>     In file included from mod-test.c:23:0:
>     ../../../src/emacs-module.h:38:47: error: missing binary operator before 
> token "("
>      #if defined __has_attribute && __has_attribute(__nonnull__)

I think the "if defined __has_attribute &&" part is not needed, since
conf_post.h defines it fr compilers that lack it.  Can you try that?