Autoconf-2.71

Reference: 8.44. Autoconf-2.71. Fix the build system:

patch -Np1 -i autoconf-2.71-bld-20220215.patch

Build and install autoconf:

./configure --prefix=/plng \
            --datarootdir=/plng/dat \
            --mandir=/plng/man \
            --infodir=/tmp

make
make install