Helpful error message for invalid -define: cases

src/main.cpp

@@ -1038,6 +1038,7 @@ bool parse_build_flags(Array<String> args) {
 							if (v.kind != ExactValue_Invalid) {
 								map_set(&build_context.defined_values, key, v);
 							} else {
+								gb_printf_err("Invalid define constant value: '%.*s'. Define constants must be a valid Odin literal.\n", LIT(value));
 								bad_flags = true;
 							}