Index: config/locale/dragonfly/c_locale.h
===================================================================
RCS file: /cvsroot/src/external/gpl3/gcc/dist/libstdc++-v3/config/locale/dragonfly/c_locale.h,v
retrieving revision 1.5
diff -p -u -r1.5 c_locale.h
--- config/locale/dragonfly/c_locale.h	11 Apr 2021 00:02:24 -0000	1.5
+++ config/locale/dragonfly/c_locale.h	23 Apr 2021 08:08:06 -0000
@@ -44,6 +44,15 @@
 #include <xlocale.h>
 #else
 #include <cstdio>
+// If __LOCALE_T_DECLARED isn't defined, then it is most likely that
+// _XOPEN_SOURCE is defined or _POSIX_C_SOURCE is defined to something
+// less than 200809L.  Deal with this.
+# ifndef __LOCALE_T_DECLARED
+typedef struct _locale          *locale_t;
+int     vsnprintf_l(char * __restrict, size_t, locale_t,
+		    const char * __restrict, __va_list) __printflike(4, 0);
+#  define __LOCALE_T_DECLARED
+# endif
 #endif
 
 #define _GLIBCXX_NUM_CATEGORIES 0