* modules/extern-inline, m4/extern-inline.m4: New files.
This is for better support of 'extern inline' a la ISO C99,
with a portable alternative on compilers that do not support
C99-style 'extern inline'. Using 'extern inline' shrinks the size
of the Emacs executable, when compiled with debugging disabled,
which is a typical way that Emacs is built while developing.
