Make getprogname() porting mandatory

Although the function is documented as possibly returning NULL if it
cannot find a known source of information, we should still at least
attempt to port it to any supported system, and otherwise explicitly
mark it as not implementable for such systems if that was to be the
case.

src/progname.c

@@ -126,6 +126,8 @@ done:
 		free(wpath);
 		free(mbname);
 	}
+#else
+#error "Function getprogname() needs to be ported."
 #endif
 
 	return __progname;