Reported upstream as https://svn.boost.org/trac/boost/ticket/2596

--- boost1.38-1.38.0.orig/tools/jam/src/jam.h
+++ boost1.38-1.38.0/tools/jam/src/jam.h
@@ -419,6 +419,10 @@
     #include <stdlib.h>
 #endif
 
+#ifdef __GLIBC__
+#  include <unistd.h>
+#endif
+
 #if !defined( OS_BSDI         ) && \
     !defined( OS_FREEBSD      ) && \
     !defined( OS_DRAGONFLYBSD ) && \