diff --git a/include/boost/config/platform/bsd.hpp b/include/boost/config/platform/bsd.hpp
index c2f61dce..8adcc4b1 100644
--- a/include/boost/config/platform/bsd.hpp
+++ b/include/boost/config/platform/bsd.hpp
@@ -46,5 +46,6 @@
 #define BOOST_HAS_GETTIMEOFDAY
 
 // boilerplate code:
+#define BOOST_HAS_UNISTD_H
 #include <boost/config/posix_features.hpp>
 
diff --git a/include/boost/config/platform/hpux.hpp b/include/boost/config/platform/hpux.hpp
index 16e5def6..5d1af3f8 100644
--- a/include/boost/config/platform/hpux.hpp
+++ b/include/boost/config/platform/hpux.hpp
@@ -19,5 +19,6 @@
 #define BOOST_NO_CWCTYPE
 
 // boilerplate code:
+#define BOOST_HAS_UNISTD_H
 #include <boost/config/posix_features.hpp>
 
diff --git a/include/boost/config/platform/irix.hpp b/include/boost/config/platform/irix.hpp
index 25ae4a53..2a991d10 100644
--- a/include/boost/config/platform/irix.hpp
+++ b/include/boost/config/platform/irix.hpp
@@ -12,5 +12,6 @@
 #define BOOST_NO_SWPRINTF 
 
 // boilerplate code:
+#define BOOST_HAS_UNISTD_H
 #include <boost/config/posix_features.hpp>
 
diff --git a/include/boost/config/platform/linux.hpp b/include/boost/config/platform/linux.hpp
index dac085e3..113694a4 100644
--- a/include/boost/config/platform/linux.hpp
+++ b/include/boost/config/platform/linux.hpp
@@ -49,6 +49,7 @@
 #endif
 
 // boilerplate code:
+#define BOOST_HAS_UNISTD_H
 #include <boost/config/posix_features.hpp>
 
 #ifndef __GNUC__
diff --git a/include/boost/config/platform/solaris.hpp b/include/boost/config/platform/solaris.hpp
index b2ab1b0c..fae4293d 100644
--- a/include/boost/config/platform/solaris.hpp
+++ b/include/boost/config/platform/solaris.hpp
@@ -12,6 +12,7 @@
 #define BOOST_HAS_GETTIMEOFDAY
 
 // boilerplate code:
+#define BOOST_HAS_UNISTD_H
 #include <boost/config/posix_features.hpp>