gethrxtime: better 'inline'

gethrxtime: better 'inline'

* lib/xtime.c: New file.
* lib/gethrxtime.c, lib/gethrxtime.h (GETHRXTIME_INLINE):
* lib/xtime.h (XTIME_INCLUDE):
New macros.  Replace all uses of 'static inline' with them.
* lib/gethrxtime.c (gethrxtime): Define only if
! (HAVE_ARITHMETIC_HRTIME_T && HAVE_DECL_GETHRTIME), since
this source file is now always compiled, because of the extern inline.
* lib/gethrxtime.h, lib/xtime.h:
Use _GL_INLINE_HEADER_BEGIN, _GL_INLINE_HEADER_END.
* m4/gethrxtime.m4 (gl_GETHRXTIME): Do not check for clock macros
if gethrtime works, as they're not needed in that case.
(gl_XTIME): Do not require AC_C_INLINE.
(gl_PREREQ_GETHRXTIME): Remove; all uses removed, as it's always
compiled now.  Move the check into gl_GETHRXTIME.
* modules/gethrxtime (Files, lib_SOURCES): Add lib/xtime.c.
(Depends-on): Add extern-inline.
(configure.ac): gethrxtime is always compiled now.
(lib_SOURCES): Add gethrxtime.c.