diff --git a/compat/crypt.h b/compat/crypt.h
index af07aa7..41c6454 100644
--- a/compat/crypt.h
+++ b/compat/crypt.h
@@ -25,17 +25,15 @@
    Encryption is not supported.
 */
 
-#ifndef _ZLIB_H
-#  ifndef ZLIB_VERNUM
+#ifndef ZLIB_VERNUM
 /* No zlib */
 typedef uint32_t z_crc_t;
-#  elif (ZLIB_VERNUM & 0xf != 0xf) && (ZLIB_VERNUM < 0x1270)
+#elif (ZLIB_VERNUM & 0xf != 0xf) && (ZLIB_VERNUM < 0x1270)
 /* Define z_crc_t in zlib 1.2.6 and less */
 typedef unsigned long z_crc_t;
-#  elif (ZLIB_VERNUM & 0xf == 0xf) && (ZLIB_VERNUM < 0x12df)
+#elif (ZLIB_VERNUM & 0xf == 0xf) && (ZLIB_VERNUM < 0x12df)
 /* Define z_crc_t in zlib-ng 2.0.7 and less */
 typedef unsigned int z_crc_t;
-#  endif
 #endif
 
 #define CRC32(c, b) ((*(pcrc_32_tab + (((int)(c) ^ (b)) & 0xff))) ^ ((c) >> 8))
diff --git a/compat/ioapi.h b/compat/ioapi.h
index c098efc..8250746 100644
--- a/compat/ioapi.h
+++ b/compat/ioapi.h
@@ -4,11 +4,11 @@
 
 #include <stdint.h>
 
-#ifndef _ZLIB_H
-#  if __has_include(<zlib.h>)
-#    include <zlib.h>
-#  elif __has_include(<zlib-ng.h>)
+#if !defined(_ZLIB_H) && !defined(ZLIB_H) && !defined(ZLIB_H_)
+#  if __has_include(<zlib-ng.h>)
 #    include <zlib-ng.h>
+#  elif __has_include(<zlib.h>)
+#    include <zlib.h>
 #  endif
 #endif