Improve build_odin.sh

[sus-impost0r]

• Fix a bug in config_darwin where a llvm-config version less than 13 would be accepted because a parameter expansion was forgotten.
• Port the script to POSIX sh (replace [[ ]] with [ ], remove set -eu)
• Quote Every Fucking Substitution consistently (except when necessary for proper fucntionality).
• Style changes: Braces around variable names. Single quotes when no substitution is required.
• Also removed unnecessary array use

[gingerBill]

First is not a bug. We do support versions 11, 12, and 13.