It doesn't, at least not in any form you'd want to rely on. Bazel makes it easier than CMake to pull in externals in source form, but you'll still want to write Bazel build rules for them in most cases. In practice for C/C++ externals being built in a Bazel project, almost all of the build can be captured by a couple

    cc_library(...)

and

    cc_binary(...)

rules.