build2 is an open source, cross-platform toolchain for building and packaging C++ code. It includes a build system, package manager, and repository web interface. There is also cppget.org, a public repository of open source C++ packages.
From the announcement:
This is the first alpha release and currently it is more of a technology preview rather than anything that is ready for production. It has been tested on various Linux'es, Mac OS, and FreeBSD. There is no Windows support yet (but cross-compilation is supported).