make sure shasum is installed before running f-c-p/clang/setup.sh
make sure shasum is installed before running f-c-p/clang/setup.sh

Summary:
public
Because we ran `./configure` *after* checking if clang was to be compiled, we
could fail mysteriously if `shasum` was not present. Now, we fail at the
`./configure` step with a clear error message.

closes #275

Reviewed By: akotulski

Differential Revision: D2921841

fb-gh-sync-id: b8edd8b
shipit-source-id: b8edd8b

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