Ensure downloads are correct via checksums

This commit adds in SHA512 and MD5 checksums for all dependencies I could find.  I'd appreciate some testing, (easiest way is to mess up the checksums put in `deps/deps_checksums.{sha512,md5}` and then rebuild the dependency whose checksum you altered) to ensure both that normal compilation continues to work, as well as altered files don't allow compilation to continue.

I tested on OSX and Linux, but didn't test on Windows or Windows cross-compile.

This PR is in reference to https://github.com/JuliaLang/julia/issues/6558