gnulib support for C99-style extern inline

[Paul Eggert]

I'm trying to get GNU Emacs to use more inline functions
instead of macros.  A problem I'm running into is that,
when compiled without optimization, 'static inline' functions
in headers cause the executable to bloat, since there's a separate
copy of the function in each module.  To fix this, I'd like to
add support for C99 'extern inline' to Emacs, when compiling with
compilers that support C99 'extern inline'.  Emacs uses a few
gnulib modules, so I'm starting by adjusting these modules to
use 'extern inline' in headers, rather than 'static inline',
in compilers that support this.

I'm about to send off a series of five proposed patches to do this.
I haven't installed them yet, to give some time for discussion.