Port cleanup attribute to OpenBSD

Port cleanup attribute to OpenBSD

The OpenBSD C compiler issues false alarms about strcpy, strcat, and
sprintf, and this messes up 'configure' when it tests for the cleanup
attribute.  Work around the problem by using __has_attribute directly.
Problem reported by Joakim Jalap (Bug#22385).
* configure.ac: Don’t use AX_GCC_VAR_ATTRIBUTE.
* m4/ax_gcc_var_attribute.m4: Remove.
* src/conf_post.h (__has_attribute): Provide a substitute, for
non-GCC or older GCC compilers.  All uses changed to assume
the substitute.  Check for the cleanup attribute.
* src/emacs-module.c (module_has_cleanup): Just use __has_attribute.