diff --git a/include/boost/config/stdlib/libstdcpp3.hpp b/include/boost/config/stdlib/libstdcpp3.hpp
index 585ee4c8..7de49e5e 100644
--- a/include/boost/config/stdlib/libstdcpp3.hpp
+++ b/include/boost/config/stdlib/libstdcpp3.hpp
@@ -54,11 +54,9 @@
 #  define BOOST_HAS_THREADS
 #endif
 
-#if !defined(CYGWIN) && (defined(unix) || defined(__unix) || defined(__unix__))
+#if !defined(CYGWIN)
 #  include <unistd.h>
-#  if defined(_POSIX_VERSION) || defined(_XOPEN_VERSION)
-#     define BOOST_HAS_FENV_H
-#  endif
+#  define BOOST_HAS_FENV_H
 #endif
 
 #if !defined(_GLIBCPP_USE_LONG_LONG) \