Can't build runtime/libc when parent path has a whitespace #987

To reproduce:

mkdir "WHITE SPACE"
cd "WHITE SPACE"
git clone git@github.com:rems-project/cerberus.git
cd cerberus
make

This will fail when building in runtime/libc/dune.

The likely solution is to apply Filename.quote to filenames that cerberus passes as arguments to a command (i.e. clang in cpp). There is probably a similar issue when passing a bad filename to -I.