Make comptime.sh conform to POSIX and less redundant, among other improvements

- Shebang now directly calls /bin/sh
- Improved indenting within the comprevision() function
- Quoted several strings to prevent possible argument splitting
- Replaced archaic backtick command statements with more descriptive `"$(...)"` statements
- Replaced direct references to `test` with more readable, but equivalent `[ ... ]` statements
- Replaced the ${str:0:8} bashism with an equivalent statement using POSIX cut
- Moved redundant definitions of the comptime.h body to a single function that takes arguments