include_next: do not check for standard headers like stddef.h

include_next: do not check for standard headers like stddef.h

I found this problem when modifying Emacs to use gnulib.
I noticed that it added HAVE_STDDEF_H to config.h, even though
gnulib always assumes <stddef.h> exists as per README and this
symbol is unnecessary.
* m4/include_next.m4 (gl_NEXT_HEADERS): New macro, which does not
use AC_CHECK_HEADERS_ONCE, but which otherwise contains what
gl_CHECK_NEXT_HEADERS used to contain.  This makes 'configure' run
faster for headers like stddef.h that are known to exist.
(gl_CHECK_NEXT_HEADERS): Use it.
* m4/float_h.m4 (gl_FLOAT_H): For float.h, use gl_NEXT_HEADERS
rather than gl_CHECK_NEXT_HEADERS.
* m4/stdarg.m4 (gl_STDARG_H): Likewise, for stdarg.h.
* m4/stddef_h.m4 (gl_STDDEF_H): Likewise, for stddef.h.