Avoid "inline" after return type in function definitions

[Paul Eggert]

Joseph Myers wrote:
> if you use a list
> of individual options instead of -Wextra, there are lots that are
> appropriate to use, and the list increases with each GCC version so you
> keep needing configure tests for new options.

Gnulib's manywarnings module does this: it tests for almost all GCC warning 
options (excluding only warnings that would seem to be harmful for pretty much 
any GNU project), and provides a way for each project to tailor the list of 
warnings, typically by omitting options that are not useful for that project. 
Because we update manywarnings with each new GCC release, downstream projects 
that use the module don't need to track GCC versions all that carefully.

manywarnings arranges for 'configure'-time tests for all the warnings, and could 
be used as a source of ideas for glibc, if we wanted to go that route.