dll feature test: output #error directive on failure

A common cause of build failures on some systems is that the output
block in the dll feature test silently fails to compile. This leads
to very-hard-to-trace compiler errors about missing identifiers
later on. iffe syntax does not allow aborting compilation if a
block does not compile, however, it does let us produce alternative
output from a shell script if compilation fails. This can be used
to generate an informative #error directive that is inserted in
place of the missing identifiers.

src/lib/libdll/features/dll:
- Add fail block to output block that produces an #error directive.

src/lib/libdll/features/dll

@@ -261,4 +261,6 @@ tst	- output{
 		printf("\n");
 		return 0;
 	}
+}end fail{
+	echo '#error The output block in src/lib/libdll/features/dll failed to compile'
 }end