Clang: it may have already defined true/false

src/doomtype.h

@@ -156,6 +156,7 @@ typedef int32_t boolean;
 #endif
 
 #ifndef __cplusplus
+#ifndef __bool_true_false_are_defined
 #ifndef _WIN32
 enum {false = 0, true = 1};
 #else
@@ -163,6 +164,7 @@ enum {false = 0, true = 1};
 #define true TRUE
 #endif
 #endif
+#endif
 
 /* 7.18.2.1  Limits of exact-width integer types */