Simplify autogen.sh.

This script is expected to perform only the autoconf specific setup,
but not run configure, make or the test suite, so remove everything
which is not related to autoconf.

People who prefer to run everything in one go can easily do so by
writing their own script, for example autogen-local.sh. Add a line
to .gitignore which ignores files matching *-local* to prevent such
files from showing up in git status.