fix: Don't pass -v to the preprocessor

fix: Don't pass -v to the preprocessor

The -v option tells the compiler to print various diagnostics to stdout,
including randomly generated filenames. When ccache calls the
preprocessor and -v is on the command line, the random information will
become part of the input hash, making the preprocessor mode always
produce cache misses.

Fix this by only passing -v when compiling, not preprocessing.

Closes #1293.