Pull out preprocessing code

Put the preprocessing code in its own small module, Pplight. This make
the main script much smaller and also makes it easier to test the
pseudo-preprocessing.
Add actual system header paths

The fixme almost said how to do it.

Some header files (e.g. stdint.h, stdarg.h) are provided by the
compiler, but in the stdint case the compiler's version contains an
#include_next directive if __STDC_HOSTED__ to use the one from the C
library. So define that macro, unless the --linux flag has been given.
Eat my own dogfood

Running check_includes on cident.c revealed that I'm not actually
using any of uint32_t or friends.
Add Makefile

This may be a bit huge for a program as small as cident, but it was
easier to cut'n'paste stuff from another project than to write a
Makefile from scratch.
Add script for testing Pplight

I haven't bothered setting up an actual test framework. Maybe some day
I'll add a test target to the Makefile.
Pplight: Add final newline if missing

Some processing is a little easier if one knows that the source
consists of properly terminated lines. So also be careful that the
backslash-newline replacing doesn't violate this.
Pplight: Handle simple #if 0

It may be reasonable to ignore blocks of code which has been #if 0'ed
out, so handle that simple case.