[Bazel] Should the toolchain set the same variables as tools/building.py?

When testing with a dummy configure script, I can see that the following variables are set correctly: `CC`, `CXX`, `AR`, `LD`

However these values are left blank, while they are set in [tools/building.py](https://github.com/emscripten-core/emscripten/blob/41c6d5ae768e3210b28a537d5943d94e498abf51/tools/building.py#L49): `NM`, `LDSHARED`, `RANLIB`, `EMSCRIPTEN_TOOLS`, `HOST_CC`, `HOST_CXX`, `HOST_CFLAGS`, `HOST_CXXFLAGS`, `PKG_CONFIG_LIBDIR`, `PKG_CONFIG_PATH`, `EMSCRIPTEN`, `PATH`, `ACLOCAL_PATH`, `CROSS_COMPILE`.

Is this intentional? I would have expected that the toolchain sets up Bazel in a way that running bare `configure` is the same as `emconfigure`.