boost::context::fcontext is not usable with boost > 16.1, disable
check for now.

Index: pdns-recursor/configure.ac
===================================================================
--- pdns-recursor.orig/configure.ac
+++ pdns-recursor/configure.ac
@@ -44,42 +44,6 @@ PDNS_CHECK_NETWORK_LIBS
 # API break in 1.52 (Nov 2012), so we only support that, and later.
 pdns_context_library="System V ucontexts"
 
-AC_DEFUN([PDNS_SELECT_CONTEXT_IMPL], [
-  AC_MSG_CHECKING([whether Boost is new enough to use the context library...])
-  if test $boost_major_version -ge 152; then
-    AC_MSG_RESULT([yes])
-    if test $boost_major_version -ge 157; then
-      BOOST_THREAD([$1])
-      m4_pattern_allow([^BOOST_THREAD_(LIBS|LDFLAGS)$])dnl
-      LIBS="$LIBS $BOOST_THREAD_LIBS"
-      LDFLAGS="$LDFLAGS $BOOST_THREAD_LDFLAGS"
-    fi
-    AC_MSG_NOTICE([checking whether the Boost context library actually links...])
-    if test $boost_major_version -ge 161; then
-      BOOST_FIND_HEADER([boost/context/detail/fcontext.hpp], [ : ], [
-        BOOST_FIND_LIB([context], [$1], [boost/context/detail/fcontext.hpp], [[]])
-      ])
-    else
-      BOOST_FIND_HEADER([boost/context/fcontext.hpp], [ : ], [
-        BOOST_FIND_LIB([context], [$1], [boost/context/fcontext.hpp], [[]])
-      ])
-    fi
-    case $boost_cv_lib_context in
-      (yes)
-        pdns_context_library="Boost context"
-        AC_MSG_NOTICE([MTasker will use the Boost context library for context switching])
-        ;;
-      *)
-        AC_MSG_NOTICE([Boost context library is missing])
-        AC_MSG_NOTICE([MTasker will use System V ucontexts for context switching])
-        ;;
-    esac
-  else
-    AC_MSG_RESULT([no])
-    AC_MSG_NOTICE([MTasker will use System V ucontexts for context switching])
-  fi
-])
-
 PDNS_CHECK_CLOCK_GETTIME
 
 boost_required_version=1.35
