Default to register_printf_specifier() if no printf hooking #defined

This allows us to build (non-./configured) external tools against
libstrongswan without explicitly specifiying the most commonly used
printf hooking function.

src/libstrongswan/printf_hook.h

@@ -26,6 +26,13 @@ typedef struct printf_hook_t printf_hook_t;
 typedef struct printf_hook_spec_t printf_hook_spec_t;
 typedef enum printf_hook_argtype_t printf_hook_argtype_t;
 
+#if !defined(USE_VSTR) && \
+	!defined(HAVE_PRINTF_FUNCTION) && \
+	!defined(HAVE_PRINTF_SPECIFIER)
+/* assume newer glibc register_printf_specifier if none given */
+#define HAVE_PRINTF_SPECIFIER
+#endif
+
 #if !defined(USE_VSTR) && \
 	(defined(HAVE_PRINTF_FUNCTION) || defined(HAVE_PRINTF_SPECIFIER))