make reconfigure ignores configure-time AUTOCONF environment variable

Suppose PATH points to an out-of date autoconf.
We can use the AUTOCONF environment variable with configure to override finding autoconf on PATH, but that variable is not remembered, so
make reconfigure fails.

# Recipe:
rm -rf build
AUTOCONF=/usr/bin/autoconf PATH="$MOLDY/bin:$PATH" bash configure ...
# configure + make succeed

make reconfigure
# Fails with:
Using autoconf at $MOLDY/bin/autoconf [autoconf (GNU Autoconf) 2.62]
stdin:33: error: Autoconf version 2.69 or higher is required