Support bash versions prior to 4.2

[danielchasehooper]

The if [ -v variable ] syntax was introduced in bash 4.2, so in order to support prior bash versions, I reverted to the if [ "$var" = "1" ] syntax.

To avoid unbound variable errors caused by set -u, I assign 0 to variables before doing argument parsing. There are pre-bash-4.2 ways to check if a variable is set without triggering the unbound variable warning (such as if [ "${clang+set}"] ) but that's pretty weird looking, so I thought it was simpler to just declare the variables ahead of time.

I realize that you removed that syntax, likely due to a combination of wanting the cleaner -v syntax, and needing to play nice with the set -u unbound variable check, but hopefully the incompatibility with bash <4.2 was an oversight and not a choice (I was just trying to build with bash 3.2)