add optimize option

build.bx

@@ -2,3 +2,4 @@ target := box
 files := src
 warnings := all extra no-unused-parameter
 std := c++14
+optimize := 3

src/toolchain.cc

@@ -173,6 +173,11 @@ void getFlags(const BXVariables &vars, FileType type, std::vector<std::string> &
 		}
 	}
 
+	auto optimize = vars.find("optimize");
+	if (optimize != vars.end()) {
+		flags.push_back("-O" + optimize->second[0]);
+	}
+
 	auto cflags = vars.find(
 			type == FileType::C ? "cflags" :
 			type == FileType::CXX ? "cxxflags" :