Index: rts/lib/fmt/os.h
--- rts/lib/fmt/os.h.orig
+++ rts/lib/fmt/os.h
@@ -13,7 +13,7 @@
 #include <cstdio>
 #include <system_error>  // std::system_error
 
-#if defined __APPLE__ || defined(__FreeBSD__)
+#if defined __APPLE__ || defined(__FreeBSD__) || defined(__OpenBSD__)
 #  include <xlocale.h>  // for LC_NUMERIC_MASK on OS X
 #endif
 
