(c-macro-preprocessor): Define a preprocessor name

(c-macro-preprocessor): Define a preprocessor name
which is valid on MS-DOS.

(c-macro-expansion): Do not rely on ``/tmp/'' being present; use
environment variables as alternatives (MSDOS).  Use
`shell-file-name' rather than ``sh''.  Redirect `stderr' with
`call-process-region' option, not from the shell.  Handle shells
which don't return exit code from `cpp' (MSDOS).  Put messages
from `cpp' inside a comment, to avoid messing up syntax highlighting.