To compile to a non-current version, I could download a tar ball, or switch

to an older branch in git, then do ./autogen.sh (or ./configure) && make 

&& make install.

But for some purpose, I may want to compile to historical version easily

without redundantly downloading the source by using some magic flag

passed into either ./configure, ./autogen.sh, or make, e.g.:

$ ./configure --version=2.0, or

$ ./autogen.sh --version-git=1.0 

Is that possible at all?

Related question, normally speaking, how to test back-compatibility

of a new patch, or do we need to test it at all?

P.S., I was just reading v0.0.1 libvirt [1].

It is so nice to see there is, well, was nothing more then 2000 LOC

for C source code.

[1] ftp://libvirt.org/libvirt/old/libvir-0.0.1.tar.gz

Dan