[BUILD] Provide Automated LLVM4 Usage on Mac

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Problem

Several users have reported problems with LLVM on MAC, see some examples:

https://github.com/EOSIO/eos/search?q=LLVM+mac&unscoped_q=LLVM+mac&type=Issues

Possible Solution

Here the most simple solution (unverified) :

#43 (comment)

The build-script should manage this "out of the box".

Resources

Use the information/script provided within #2392 (comment), more concretely:

https://gist.github.com/tpmccallum/45eb7d34a5406508419fa7e676909006

Requirements

(draft, unverified)

• do not force users to uninstall newer LLVM versions
• install LLVM4 and apply all necessary changes

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