use boost::async instead of std::async as ~boost::async is nonblocking

configure.ac

@@ -143,6 +143,17 @@ PKG_CHECK_MODULES([ICU], [icu-uc], [
 	)
 ], [[]])
 
+dnl =============================
+dnl = checking for boost.thread =
+dnl =============================
+AC_DEFINE([BOOST_THREAD_VERSION], [3], [require boost.thread v3])
+AC_CHECK_HEADERS([boost/thread.hpp], ,
+	AC_MSG_ERROR([boost/thread.hpp is missing or your boost version is too old (boost.thread v3 is required)])
+)
+AC_CHECK_LIB(boost_thread$BOOST_LIB_SUFFIX, main, LIBS="$LIBS -lboost_thread$BOOST_LIB_SUFFIX",
+	AC_MSG_ERROR([no boost.thread library found])
+)
+
 dnl ================================
 dnl = checking for various headers =
 dnl ================================