Export major version of latest Emacs supported by emacs-module.h.

Export major version of latest Emacs supported by emacs-module.h.

This is useful if module authors want to support multiple versions of
emacs-module.h.

* configure.ac (emacs_major_version): Define substitution.

* src/emacs-module.h.in (EMACS_MAJOR_VERSION): Define macro.

* doc/lispref/internals.texi (Module Initialization): Document
EMACS_MAJOR_VERSION preprocessor macro.

* test/data/emacs-module/mod-test.c (emacs_module_init): Verify
behavior of EMACS_MAJOR_VERSION.