Building mail-filter/opendkim without this patch would fail because
mfapi.h doesn't include libmilter's own config header which defines
SM_CONF_STDBOOL_H.

We certainly only care about >= C99 compilers so we can just always
take that path.

https://bugs.gentoo.org/944138
--- a/include/libmilter/mfapi.h
+++ b/include/libmilter/mfapi.h
@@ -89,7 +89,7 @@ typedef int	sfsistat;
 # endif
 #endif /* __P */
 
-#if SM_CONF_STDBOOL_H
+#if 1 /* SM_CONF_STDBOOL_H */
 # include <stdbool.h>
 #else /* SM_CONF_STDBOOL_H */
 # ifndef __cplusplus