Add dynamic module module support

Add dynamic module module support

* configure.ac: Add '--with-modules' option.  Conditionally add
  dynlib.o and module.o to the list of objects.  Add any system
  specific flags to the linker flags to support dynamic libraries.
* m4/ax_gcc_var_attribute.m4: Add autoconf extension to test gcc
  attributes.
* src/Makefile.in: Conditionally add module objects and linker flags.
* src/alloc.c (garbage_collect_1): protect module local values from
  GC.
* src/lisp.h: Add 'module_init' and 'syms_of_module' prototypes.
* src/emacs_module.h: New header file included by modules.  Public
  module API.
* src/module.c: New module implementation file.

Co-authored-by: Philipp Stephani <phst@google.com>