CMAKE_RANLIB should be `:` in certain scenarios, instead of `""`

[gr1mpatr0n]

rules_foreign_cc/foreign_cc/private/cmake_script.bzl

Line 148 in 0bcec88

params.cache.update({"CMAKE_RANLIB": ""})

In specific scenarios, CMAKE_RANLIB will be injected into command lines which are attempting to use the empty string as a command, breaking (e.g. "" libjansson.a). I'm not entirely sure when this is a problem, but I've seen it primarily when cross-compiling for Windows using clang-cl. This appears to be corroborated by the OpenCV project, which uses CMAKE_RANLIB=: for those builds.

I'd like to understand why this happens though...

[gr1mpatr0n]

Prior art: A cursory search for RANLIB=: on GitHub shows that it is a thing and has been in autoconf etc. since forever (https://github.com/search?q=RANLIB%3D%3A&type=code).

I'm therefore not sure why my tests are failing...