Allow to build with clang-cl

include/msvc_compat/C99/stdbool.h

@@ -5,7 +5,11 @@
 
 /* MSVC doesn't define _Bool or bool in C, but does have BOOL */
 /* Note this doesn't pass autoconf's test because (bool) 0.5 != true */
+/* Clang-cl uses MSVC headers, so needs msvc_compat, but has _Bool as
+ * a built-in type. */
+#ifndef __clang__
 typedef BOOL _Bool;
+#endif
 
 #define bool _Bool
 #define true 1