Make sure that SYSCONFDIR is always set

This is useful when ./configure hasn't been run but some source code
linters are run in an editor, for example.

src/portab/portab.h

@@ -175,6 +175,10 @@ extern int vsnprintf PARAMS(( char *str, size_t count, const char *fmt, va_list
 # define PACKAGE_VERSION VERSION
 #endif
 
+#ifndef SYSCONFDIR
+# define SYSCONFDIR "/etc"
+#endif
+
 #endif
 
 /* -eof- */