diff --git a/zconf.h b/zconf.h
index c9fc52b..bf977d3 100644
--- a/zconf.h
+++ b/zconf.h
@@ -473,11 +473,18 @@ typedef uLong FAR uLongf;
 #  undef _LARGEFILE64_SOURCE
 #endif
 
-#if defined(__WATCOMC__) && !defined(Z_HAVE_UNISTD_H)
-#  define Z_HAVE_UNISTD_H
+#ifndef Z_HAVE_UNISTD_H
+#  ifdef __WATCOMC__
+#    define Z_HAVE_UNISTD_H
+#  endif
+#endif
+#ifndef Z_HAVE_UNISTD_H
+#  if defined(_LARGEFILE64_SOURCE) && !defined(_WIN32)
+#    define Z_HAVE_UNISTD_H
+#  endif
 #endif
 #ifndef Z_SOLO
-#  if defined(Z_HAVE_UNISTD_H) || defined(_LARGEFILE64_SOURCE)
+#  if defined(Z_HAVE_UNISTD_H)
 #    include <unistd.h>         /* for SEEK_*, off_t, and _LFS64_LARGEFILE */
 #    ifdef VMS
 #      include <unixio.h>       /* for off_t */
diff --git a/zconf.h.cmakein b/zconf.h.cmakein
index 8e4b5bd..247ba24 100644
--- a/zconf.h.cmakein
+++ b/zconf.h.cmakein
@@ -475,11 +475,18 @@ typedef uLong FAR uLongf;
 #  undef _LARGEFILE64_SOURCE
 #endif
 
-#if defined(__WATCOMC__) && !defined(Z_HAVE_UNISTD_H)
-#  define Z_HAVE_UNISTD_H
+#ifndef Z_HAVE_UNISTD_H
+#  ifdef __WATCOMC__
+#    define Z_HAVE_UNISTD_H
+#  endif
+#endif
+#ifndef Z_HAVE_UNISTD_H
+#  if defined(_LARGEFILE64_SOURCE) && !defined(_WIN32)
+#    define Z_HAVE_UNISTD_H
+#  endif
 #endif
 #ifndef Z_SOLO
-#  if defined(Z_HAVE_UNISTD_H) || defined(_LARGEFILE64_SOURCE)
+#  if defined(Z_HAVE_UNISTD_H)
 #    include <unistd.h>         /* for SEEK_*, off_t, and _LFS64_LARGEFILE */
 #    ifdef VMS
 #      include <unixio.h>       /* for off_t */
diff --git a/zconf.h.in b/zconf.h.in
index c9fc52b..bf977d3 100644
--- a/zconf.h.in
+++ b/zconf.h.in
@@ -473,11 +473,18 @@ typedef uLong FAR uLongf;
 #  undef _LARGEFILE64_SOURCE
 #endif
 
-#if defined(__WATCOMC__) && !defined(Z_HAVE_UNISTD_H)
-#  define Z_HAVE_UNISTD_H
+#ifndef Z_HAVE_UNISTD_H
+#  ifdef __WATCOMC__
+#    define Z_HAVE_UNISTD_H
+#  endif
+#endif
+#ifndef Z_HAVE_UNISTD_H
+#  if defined(_LARGEFILE64_SOURCE) && !defined(_WIN32)
+#    define Z_HAVE_UNISTD_H
+#  endif
 #endif
 #ifndef Z_SOLO
-#  if defined(Z_HAVE_UNISTD_H) || defined(_LARGEFILE64_SOURCE)
+#  if defined(Z_HAVE_UNISTD_H)
 #    include <unistd.h>         /* for SEEK_*, off_t, and _LFS64_LARGEFILE */
 #    ifdef VMS
 #      include <unixio.h>       /* for off_t */