Apply stable-release fixes from 2017 to older autoconf releses:
- Fix the scripts autoheader and autoscan to pass the test suite
- Fix test case to passing when libtool 2.4.3+ is in use
autoconf-2.13 dates back to 1999. The build wasn't possible since
4 years: Since 2017, we patch autom4te which didn't exist in 2.13,
failing the build of it. 4 years of not being able to build 2.13
is a crystal clear indication that we can remove it safely.
fd111a3395