Changed cf-define to cf-subst, to align AC_SUBST

For autoconf, C preprocessor definitions are the main thing and
AC_DEFINE is used for that.  For our purpose, though, @NAME@ substituion
is more useful and important, so we used to name it as cf-define;
however, to avoid confusion, we decided to follow autoconf naming convention
strictly.
Added cf-define to add DEFS, like AC_DEFINE
Rewrote 'warn' in Scheme, obsoleting Scm_FWarn()
Improved argument processing in gauche.configure