Try to avoid future regressions on the VPATH front.
* autobuild.sh: Uncomment VPATH use.
* .gitignore: Exclude build directory.
---
v2, not that much changed since:
https://www.redhat.com/archives/libvir-list/2010-April/msg01422.html
but now that all the intermediate VPATH issues that had stumped me
for so long have been resolved, it finally works!