Add path option (-SI <path>) for #include <file>

Now system files (the ones included using angle brackets, e.g. #include
<stdio.h>) can have their own path. Specify it with -SI path. Just like
the -I option, the -SI option can be repeated multiple times to specify
multiple search paths.

To sum it up:

"file" is first searched in the current directory (which may differ from
the directory of file that does #include "file"), then in the paths
provided by the -I option, then in the paths provided by the -SI option.

<file> is searched in the paths provided by the -SI option.