Spurious buffer saving triggered by M-x compile

[Geoff Gole]

At the moment, M-x compile and friends will trigger the saving of
every file-backed buffer. When there are unsaved buffers unrelated to
the compilation, this becomes quite annoying.

What does everybody think about adding a way to exclude buffers from
this process? We could allow compilation-ask-about-save to be a
predicate function, or add a variable
compilation-save-buffer-predicate.