m4: Update m4/threadlib.m4 from gnulib.
* m4/threadlib.m4: Update.
Copied from gnulib commit of 12b5b00f93, so that we can support build
with GNU C library 2.34 or later.
Parts of our local changes are not needed any more:
- Fix comment for "http" to "https"
- Update to AS_HELP_STRING for newer autoconf
One of our local change is canceled by this commit:
- CPPFLAGS handling by THREADLIB_CPPFLAGS
And it will be merged by following commit.
- GnuPG-bug-id: T5547
- Signed-off-by: NIIBE Yutaka <firstname.lastname@example.org>