Avoid passing compilation-only options to the preprocessor redux

Avoid passing compilation-only options to the preprocessor redux

25e73c1f (“Include compiler-only arguments in the hash”) attempted to
fix a regression in 5d8585b5 (“Don’t pass -Werror and compilation-only
options to the preprocessor”). It succeeded fixing the regression all
right, but it also essentially reverted the essence of 5d8585b5 (#312)
since compiler-only arguments once again are passed to the preprocessor.
Sigh.

Fix this for real and also write a test that proves it.

(cherry picked from commit bc2005e994030f787c014ce2491cfc4c7057eac6)