diff --git a/frmts/zlib/zconf.h b/frmts/zlib/zconf.h
index ede3c82e3e..d53268ade1 100644
--- a/frmts/zlib/zconf.h
+++ b/frmts/zlib/zconf.h
@@ -433,11 +433,13 @@ typedef uLong FAR uLongf;
    typedef unsigned long z_crc_t;
 #endif
 
-#if 1    /* was set to #if 1 by ./configure */
+/* Patched by GDAL */
+#ifdef HAVE_UNISTD_H    /* may be set to #if 1 by ./configure */
 #  define Z_HAVE_UNISTD_H
 #endif
 
-#if 1    /* was set to #if 1 by ./configure */
+/* Patched by GDAL */
+#ifdef HAVE_STDARG_H
 #  define Z_HAVE_STDARG_H
 #endif
 
@@ -540,4 +542,9 @@ typedef uLong FAR uLongf;
   #pragma map(inflate_copyright,"INCOPY")
 #endif
 
+/* Patched by GDAL */
+#ifdef RENAME_INTERNAL_ZLIB_SYMBOLS
+#include "gdal_zlib_symbol_rename.h"
+#endif
+
 #endif /* ZCONF_H */