* libguile.h: #include "__scm.h" before testing the STDC_HEADERS

* libguile.h: #include "__scm.h" before testing the STDC_HEADERS
preprocessor symbol; "__scm.h" is where it might get #defined.
* __scm.h: Similar: #include <scmconfig.h> before testing
HAVE_LIMITS_H.

* __scm.h: It's HAVE_LIMITS_H, not HAVE_LIMITSH.

* __scm.h (PROTO): New macro, for declaring functions with
prototypes.

* __scm.h (SCM_SYSMISSING): When we don't have ENOSYS, don't
complain about it in the error message; the error message is
adequate without that note, and there's nothing the user can do
about it.