This incorporates:
2020-01-18 Rename ~~gnulib.m4 to zzgnulib.m4
2020-01-18 Fix "m4_require: circular dependency of AC_LANG_COMPILER(C)"
2020-01-18 Ensure Automake does not drop ~~gnulib.m4
2020-01-18 Fix major regression from 2020-01-10
* m4/00gnulib.m4, m4/gnulib-common.m4: Copy from Gnulib.
* m4/gnulib-comp.m4: Regenerate.
* m4/zzgnulib.m4: New file, from Gnulib.