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,
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 an upstream
branch which has had the embedded sources removed.  The resulting upstream
tarball is called libmongoc_<upstream-version>+dfsg.orig.tar.gz.
