int(ceil(log10(100))) is one lower than it should be when max widths are
powers of 10. This means providing a value of 100 when the max is 100
produces an unexpected failure.
Was never triggered because imghdrlen args are only hard-coded
constants, but a bug is a bug.
Debian use /usr/lib/python*/dist-packages (instead of site-packages) for
packaged python modules. It's achieved using --install-layout=deb
option, so pass it on Debian build.
