Only replace getopt on Windows, not any other platform

The preprocessor test was wrong, causing this code to be built (and potentially
replace the system getopt/long) on all platforms. This made it impossible to
build with -Werror=writable-strings on Unix without modifying this file further
to fix its casting of static strings to mutable pointers.

lib/win32/getopt_long.cpp

@@ -68,7 +68,7 @@
 
 #include "box_getopt.h"
 
-#ifdef REPLACE_GETOPT // until end of file
+#if REPLACE_GETOPT // until end of file
 
 int	opterr = 1;		/* if error message should be printed */
 int	optind = 1;		/* index into parent argv vector */