read_config.c: define SYSCONFDIR as ".", if not already defined.

Just to be safe. (and would help with cases where one might use
standalone makefiles and not autotools.)

src/read_config.c

@@ -17,6 +17,10 @@
 #include <sys/unistd.h>
 #endif
 
+#if !defined(SYSCONFDIR)
+#define SYSCONFDIR "."
+#endif
+
 static char driver[32];
 static char instrument_path[256];