Creating "private" header name space

[Mikael Djurfeldt <mdj at mdj dot nada dot kth dot se>]

A user has had the problem that system headers include Guile headers
instead of system headers.

A user application is supposed to include that header using

  #include <libguile.h>

    or

  #include <libguile/random.h>

Here there's no danger of confusion.  The confusion occurs when
compiling libguile itself.  Then the compiler is given -I. and -I
.../libguile options so that e.g. the system header <random.h> can be
confused with libguile/random.h.

I suggest that we consequently refer to Guile headers using
libguile/...  E.g., let hash.c #include "libguile/vectors.h" instead
of "vectors.h" and remove the -I. option when compiling inside the
libguile directory.  (The -I.. option will do the job.)

This way, the "libguile/..." header name space becomes more private,
so that it isn't confused with other header names.

Comments?