compiler: fix interface generation for unused interfaces

vlib/compiler/cgen.v

@@ -462,25 +462,34 @@ fn (v &V) interface_table() string {
 		if t.cat != .interface_ {
 			continue
 		}
+		interface_name := t.name
 		mut methods := ''
 		sb.writeln('// NR methods = $t.gen_types.len')
 		for i, gen_type in t.gen_types {
-			methods += '{'
+			gen_concrete_type_name := gen_type.replace('*', '')
+			methods += '{\n'
 			for j, method in t.methods {
 				// Cat_speak
-				methods += '${gen_type}_${method.name}'
+				methods += ' (void*)    ${gen_concrete_type_name}_${method.name}'
 				if j < t.methods.len - 1 {
-					methods += ', '
+					methods += ', \n'
 				}
 			}
-			methods += '}, '
-			// Speaker_Cat_index = 0
-			sb.writeln('int _${t.name}_${gen_type}_index = $i;')
+			methods += '\n},\n\n'
+			// Speaker_Cat_index = 0      
+			concrete_type_name := gen_type.replace('*', '_ptr')			
+			sb.writeln('int _${interface_name}_${concrete_type_name}_index = $i;')
 		}
+		
 		if t.gen_types.len > 0 {
 			// methods = '{TCCSKIP(0)}'
 			// }
-			sb.writeln('void* (* ${t.name}_name_table[][$t.methods.len]) = ' + '{ $methods }; ')
+			sb.writeln('void* (* ${interface_name}_name_table[][$t.methods.len]) = ' + '{ \n $methods \n }; ')
+		}else{
+			// The line below is needed so that C compilation succeeds,
+			// even if no interface methods are called.
+			// See https://github.com/zenith391/vgtk3/issues/7
+			sb.writeln('void* (* ${interface_name}_name_table[][1]) = ' + '{ {NULL} }; ')
 		}
 		continue
 	}