Prebuilt binaries for libbenchmark might be incompatible with some
environments, for example, when mbgl-benchmark target is compiled
on host with GCC and linked against mason's clang 5.0 compiled binaries.
Vendored libbenchmark avoids aforementioned issue, as the target and
the binary compiled with same compiler / options.
Vendorize the submodules and a simple CMake build
system for all them. The dependencies will inherit
compilation options for core.
The goal is to make Mapbox GL Core completely self
contained with a simple offline build.
