fix checking CXX11 when configuring

configure.ac

@@ -84,7 +84,7 @@ AC_OPENMP
 #c++11 detection
 AX_CXX_COMPILE_STDCXX(11,noext,optional)
 
-AC_SUBST([STARPU_HAVE_CXX11], [test "$HAVE_CXX11" -eq 1])
+AC_SUBST([STARPU_HAVE_CXX11], $HAVE_CXX11)
 AM_CONDITIONAL([STARPU_HAVE_CXX11], [test "$HAVE_CXX11" -eq 1])
 if test $HAVE_CXX11 -eq 1; then
   AC_DEFINE(STARPU_HAVE_CXX11, [1], [compiler supports cxx11])