Questions about checking out v2.5.1

[Kai Yu]

Hi, I'm currently investigating how to fight software regressions with git
bisect. I git clone grep and type "git checkout v2.6" to switch to the
version 2.6 and the compilation is successful. However, when I use "git
checkout v2.5.1" to switch to the version 2.5.1, after type "sh autogen.sh"
and "./configure", an error arises:

...
configure: creating ./config.status
config.status: error: cannot find input file: Makefile.in

Therefore, I cannot use "make" to compile the package. Could anybody tell me
how to resolve it?

I would appreciate any replies on this issue, thank you.