ifdef __GNUC__ is needed to support GCC 4 on Precise and Trusty used by
BUILD_APACHE_BUNDLE task:
https://a.yandex-team.ru/arc/trunk/arcadia/sandbox/projects/BuildApacheBundle/__init__.py?rev=5292353#L36

--- a/zconf.h
+++ b/zconf.h
@@ -433,8 +433,16 @@ typedef uLong FAR uLongf;
 
-#if 1    /* was set to #if 1 by ./configure */
+#ifdef __GNUC__
 #  define Z_HAVE_UNISTD_H
+#else
+#  if __has_include(<unistd.h>)
+#    define Z_HAVE_UNISTD_H
+#  endif
 #endif
 
-#if 1    /* was set to #if 1 by ./configure */
-#  define Z_HAVE_STDARG_H
+#ifdef __GNUC__
+#  define Z_HAVE_UNISTD_H
+#else
+#  if __has_include(<stdarg.h>)
+#    define Z_HAVE_STDARG_H
+#  endif
 #endif