Make build.rs compatible with Bazel's sandbox

[AlexDenisov]

When building with Bazel's sandbox, all the capstone source files are actually symlinks. In this case build.rs doesn't find any files to build and attempts to link the static library right away.

Reproducer and the error messages can be found here.
I tested the change locally on macOS with the following spec:

crate.spec(
    package = "capstone-sys",
    git = "https://github.com/AlexDenisov/capstone-rs.git",
    branch = "bazel-compatibility",
)

which is working as expected.

Is there anything else I'd need to check to ensure this change doesn't break anything?