proper edit-build-install-test cycle with v3 source packages?

[Timo Juhani Lindfors <timo.lindfors@iki.fi>]

If I hit a bug in a v1 source package I often do

apt-get source foo
cd foo-*
DEB_BUILD_OPTIONS="noopt debug nostrip" dpkg-buildpackage -rfakeroot -us -uc
sudo debi ../foo*.changes

and then run the program inside M-x gdb RET of emacs. If I want to
test a possible fix I then just use the same emacs to modify files and
then rerun dpkg-buildpackage and debi to test my changes.

The above is not really usable with v3 source packages: apparently
dpkg-buildpackage modifies the timestamps of source files and emacs
complains that these files have changed on the disk. Should I just
modify emacs to ignore this warning?

I asked my question on #debian-devel and was given the suggestion that
I should copy the source tree to a different directory for build. This
works but is slow and also has the problem that M-x gdb will open
files of the build directory instead of the original one. If I then
accidentally modify something in the build directory it will get
overwritten by the build.