Forcing XPG5 removes locale_t from <locale.h>.
This breaks including libintl.h, since gettext is built with locale_t
in view.

Index: config.mk
--- config.mk.orig
+++ config.mk
@@ -29,7 +29,7 @@ CPPFLAGS += -DGSEAL_ENABLE
 CPPFLAGS += -DGTK_DISABLE_SINGLE_INCLUDES
 CPPFLAGS += -DGDK_DISABLE_DEPRECATED
 
-ifeq "$(findstring $(OS),FreeBSD DragonFly)" ""
+ifeq "$(findstring $(OS),FreeBSD DragonFly OpenBSD)" ""
 CPPFLAGS += -D_XOPEN_SOURCE=500
 CPPFLAGS += -D__BSD_VISIBLE
 endif
