Make it possible to build in a separate directory

This patch allows you to do:
mkdir build
cd build
../configure ...
make

This will output all generated files to the build directory. Given that
autogen automatically runs configure, you can also do:

BUILDDIR=./build ./autogen.sh

which will do the right thing.

Also:

* Fix a dependency bug which means that guestfs_protocol.h
  isn't automatically rebuilt.
* Re-running autogen.sh with no arguments won't blow away your previous
  configure arguments.