Add configure.ac.stub - it is needed by modules/build_modules.sh

Delete configure.ac as it is generated by modules/build_modules.sh.

Note: git see this as renaming configure.ac to configure.ac.stub since
the difference between the two files is very small - configure.ac
has the expansion of %modules% in configure.ac.stub.

Run the following to see the diff:

	$ diff -u <(
		git show f3a230e122b2baa931b0a7934b5428afa13fbaad:configure.ac
	) configure.ac.stub
Delete modules-config.m4 as it is generated by modules/build_modules.sh
Teach autogen.sh how to create configure.ac