Restore ability to use a C++ compiler that does not claim to support C++11.

diff --git a/configure.ac b/configure.ac
index c1a8a59abbbf04bdedba802d2ca276ca2fa0e04d..8a81007cc875bbeff75ffdc22b29a9a54fc68e66 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -90,7 +90,7 @@ AC_SUBST(CL_VERSION)
 dnl           checks for compiler characteristics
 dnl
 AC_LANG([C++])
-AX_CXX_COMPILE_STDCXX([11])
+AX_CXX_COMPILE_STDCXX([11], [ext], [optional])
 dnl
 CL_LONGLONG
                       dnl DEFS HAVE_LONGLONG