add defines variable

src/toolchain.cc

 		}
 	}
 
+	auto defines = vars.find("defines");
+	if (defines != vars.end()) {
+		for (auto &d: defines->second) {
+			flags.push_back("-D" + d);
+		}
+	}
+
 	auto sanitizers = vars.find("sanitizers");
 	if (sanitizers != vars.end()) {
 		for (auto &s: sanitizers->second) {