compiler.txt: ensure file exists before comparison

[fauxpark]

Description

When CC is clang, and compiler.txt does not exist, the following is printed to stderr:

error: write on a pipe with no reader

On macOS at least, this results in false [WARNINGS] when running the unit tests.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).