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'.