Cygwin appears not to have sigaction all the time after all.

include/boost/config/platform/cygwin.hpp

@@ -23,7 +23,7 @@
 #  define BOOST_HAS_SCHED_YIELD
 #  define BOOST_HAS_GETTIMEOFDAY
 #  define BOOST_HAS_PTHREAD_MUTEXATTR_SETTYPE
-#  define BOOST_HAS_SIGACTION
+//#  define BOOST_HAS_SIGACTION
 #else
 #  if !defined(BOOST_HAS_WINTHREADS)
 #     define BOOST_HAS_WINTHREADS
@@ -51,7 +51,6 @@
 #ifdef BOOST_HAS_NL_TYPES_H
 #  undef BOOST_HAS_NL_TYPES_H
 #endif
-