diff --git a/zconf.h b/zconf.h
index 4a52aed..d6c159a 100644
--- a/zconf.h
+++ b/zconf.h
@@ -235,10 +235,12 @@
 #  endif
 #endif
 
-#if defined(ZLIB_CONST) && !defined(z_const)
-#  define z_const const
-#else
-#  define z_const
+#ifndef z_const
+#  ifdef ZLIB_CONST
+#    define z_const const
+#  else
+#    define z_const
+#  endif
 #endif
 
 #ifdef Z_SOLO
diff --git a/zconf.h.cmakein b/zconf.h.cmakein
index 7a35e8f..f43a904 100644
--- a/zconf.h.cmakein
+++ b/zconf.h.cmakein
@@ -237,10 +237,12 @@
 #  endif
 #endif
 
-#if defined(ZLIB_CONST) && !defined(z_const)
-#  define z_const const
-#else
-#  define z_const
+#ifndef z_const
+#  ifdef ZLIB_CONST
+#    define z_const const
+#  else
+#    define z_const
+#  endif
 #endif
 
 #ifdef Z_SOLO
diff --git a/zconf.h.in b/zconf.h.in
index 4a52aed..d6c159a 100644
--- a/zconf.h.in
+++ b/zconf.h.in
@@ -235,10 +235,12 @@
 #  endif
 #endif
 
-#if defined(ZLIB_CONST) && !defined(z_const)
-#  define z_const const
-#else
-#  define z_const
+#ifndef z_const
+#  ifdef ZLIB_CONST
+#    define z_const const
+#  else
+#    define z_const
+#  endif
 #endif
 
 #ifdef Z_SOLO