Log Message:
autoconf: update to 2.71.

* Noteworthy changes in release 2.71 (2021-01-28) [stable]

** Bug fixes, including:

*** Compilers that support C99 but not C2011 are detected correctly.

*** Compatibility improved with clang and Oracle C++.

*** Compatibility restored with automake's rules for regenerating configure.

*** Compatibility restored with old versions of std-gnu11.m4.

  Packages that wish to maintain compatibility with Autoconf 2.69 or
  older, should update their copy of std-gnu11.m4 from Gnulib as soon
  as practical, as the compatibility code bulks up the configure script.

  Packages that require Autoconf 2.70 can drop this file entirely.