xalloc: better 'inline'

xalloc: better 'inline'

* lib/xmalloc.c, lib/xalloc.h (XALLOC_INLINE):
New macro.  Replace all uses of 'static inline' with it.
(static_inline): Remove.
* lib/xalloc.h (xnmalloc, xnrealloc, x2nrealloc, xcharalloc):
Let 'extern inline' do the work automatically, instead of doing
it by hand.
* m4/xalloc.m4 (gl_PREREQ_XALLOC, gl_PREREQ_XMALLOC):
Remove.  All uses removed.
* modules/xalloc (Depends-on): Remove 'inline'.  Add 'extern-inline'.