Subversion is portable. It's built on top of the Apache Portable Runtime (APR) library. XXX more to go here
Hg is very portable as it is 95% Python and 5% C for speed. Hg also runs on Windows.
git is mostly a collection of C and shell, and has been ported to just about everything. There are a couple of tools in perl that may be of interest for the base system (which we would have to rewrite), a few in python that won't be, and one in tk that is definitely out. So, we shouldn't have any modula3-like issues, and should also be able to avoid bringing in any new scripting languages, including perl.
Supported (also see monotone:FeaturePortable)
Monotone is implemented in C++, and compiles and runs on all sorts of different kinds of Unix systems and Windows. It is packaged in a number of linux distribution and packaging systems, as well as NetBSD pkgsrc which itself supports a number of different systems.
Its latest release is kept updated in the ports and currently requires devel/boost, converters/libiconv and devel/gettext libraries to run.