libcdecl: Add _Bool fallback to public header.

Even though we have a fallback for _Bool when building the library,
it is missing in some source files that include cdecl.h.  But more
importantly, it is not available to downstream users of the installed
header file.

As far as I'm aware, all compilers which support _Bool in any mode also
support it in strict C89 mode, so it should be sufficient to just use
_Bool if configure detected support for it.  We integrate the newfangled
bake-config script so that the installed header embeds this config.

There is also one inline function which returns _Bool.  We can just
change it to return int instead, since there is no ABI implication.