--- a/zconf.h.cmakein
+++ b/zconf.h.cmakein
@@ -435,7 +435,7 @@
    typedef unsigned long z_crc_t;
 #endif
 
-#ifdef HAVE_UNISTD_H    /* may be set to #if 1 by ./configure */
+#if defined(HAVE_UNISTD_H) && HAVE_UNISTD_H /* FIX198: config.h may define it as 0 on Windows. */
 #  define Z_HAVE_UNISTD_H
 #endif