Using Perl SHA256 if if shasum/sha256sum unavailable

[chapmajs]

First, thanks for a great project! It's incredible that folks are still supporting OS X 10.4!

I did a clean install of OS X 10.4 on an ALS iMac G5 today, updated to 10.4.11, installed XCode 2.5, and tried to bootstrap tigerbrew. I ran into issues with brew doctor after initial download: I couldn't install the vendored Ruby or cURL due to a SHA checksum mismatch. I tracked this down to the install-vendor.sh script, which had a conditional to first look for OS-provided SHA binaries, then try a Ruby implementation if that failed, or use a Perl implementation if we're on a specific subset of PPC chips.

Apparently the G5 in the ALS iMacs (970/970fx) are also one of the CPUs that Ruby 1.8.2 has a bad SHA256 on. My sysctl -n hw.cpusubtype is 100, which is not in the excluded list. Rather than trying to figure out every subtype that has a broken SHA256, why don't we just always use the Perl implementation?

[mistydemeo]

Thanks for this PR! Really appreciate the additional info.

Unfortunately the pure-Perl implementation is much slower than the Ruby one. Thankfully most users only need to run this occasionally, but falling back to Perl would significantly slow down the vendor process for users on older CPUs. For that reason, I think trying to enumerate which versions are compatible is still useful.

[chapmajs]

OK, makes sense to me! I updated the CPU exclusion list to exclude the 970/970fx and force-pushed the change to my sha256_fix branch.