Improve compiler comparison script

[spiessimon]

This PR makes a few minor improvements to the compiler comparison script:

• Adds more documentation for the environment variables that can be set.
• Adds support for an environment variable to set autoconf.
• Ensures that the git arguments refer to valid references.
• Ensures that the target directory for the comparison is not root.
• Explicitly asks before emptying the target directories for the resulting compilers.
• Removes the temporary folder that is used for building upon exit.
• Uses a single install directory to make sure that the resulting assembly files match.
• Tweaks the copying to have the intended behavior on all systems.