While preprocessing ccache adds -E to the command line options but does not eliminate the -c
so the command line looks like
cc -c -E input.c

For gcc this is ok as it groks -c and -E together, but other compilers consider -c and -E as conflicting as -c means compile to object and -E stop after cpp.

if -c is eliminated things still work for gcc
gcc -E input.c gives the same output as gcc -c -E input.c