The upstream libmongoc bundles zLib for compression support on systems where it
is not available.  The Debian package does not build using this embedded code.
Instead it depends on zlib1g-dev.  In order to prevent issues with shipping an
embedded copy of source code from another package, the upstream source is
modified to remove the bundled zlib sources.  This is accomplished by using the
--git-prebuild option of git-buildpackage to handle generation of the upstream
tarball without the embedded code.  See debian/gbp.conf for details.