cgen: dont generate function if its flag is false (#11021)

vlib/v/ast/ast.v

@@ -398,6 +398,7 @@ pub mut:
 	return_type       Type
 	return_type_pos   token.Position // `string` in `fn (u User) name() string` position
 	has_return        bool
+	should_be_skipped bool
 	//
 	comments      []Comment      // comments *after* the header, but *before* `{`; used for InterfaceDecl
 	next_comments []Comment // coments that are one line after the decl; used for InterfaceDecl

vlib/v/checker/checker.v

@@ -7983,7 +7983,7 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 	} else {
 		for mut a in node.attrs {
 			if a.kind == .comptime_define {
-				c.evaluate_once_comptime_if_attribute(mut a)
+				node.should_be_skipped = c.evaluate_once_comptime_if_attribute(mut a)
 			}
 		}
 	}

vlib/v/gen/c/fn.v

@@ -31,6 +31,9 @@ fn (mut g Gen) process_fn_decl(node ast.FnDecl) {
 	if !g.is_used_by_main(node) {
 		return
 	}
+	if node.should_be_skipped {
+		return
+	}
 	if g.is_builtin_mod && g.pref.gc_mode == .boehm_leak && node.name == 'malloc' {
 		g.definitions.write_string('#define _v_malloc GC_MALLOC\n')
 		return