Add unistd.h and make it configure script independent

[arnabanimesh]

No description provided.

[Neustradamus]

@madler: Have you seen this PR?

[guidomodarelli]

Add this change in gzlib.c

diff --git a/gzlib.c b/gzlib.c
index 55da46a..f1d85c3 100644
--- a/gzlib.c
+++ b/gzlib.c
@@ -4,6 +4,11 @@
  */
 
 #include "gzguts.h"
+#if defined( unix ) || defined( __unix ) || defined( __unix__ ) || \
+    (defined( __APPLE__ ) && defined( __MACH__ )) || \
+    HAVE_UNISTD_H
+#include <unistd.h>         /* lseek, read, write, close, chown, unlink, sleep, ftruncate, fsync */
+#endif
 
 #if defined(_WIN32) && !defined(__BORLANDC__)
 #  define LSEEK _lseeki64

[arnabanimesh]

@guidomodarelli done

[guidomodarelli]

LGTM 🟢