force c++11 in GNU autotools
diff --git a/configure.ac b/configure.ac
index e04891f..9885e1b 100755
--- a/configure.ac
+++ b/configure.ac
@@ -102,10 +102,7 @@
AC_PROG_RANLIB
AC_LANG([C++])
-AX_CXX_COMPILE_STDCXX_11([noext], [optional])
-if test "$ac_success" = "no"; then
- CXXFLAGS="$CXXFLAGS -Wno-variadic-macros -Wno-long-long -Wno-c++11-long-long"
-fi
+AX_CXX_COMPILE_STDCXX_11([noext], [mandatory])
AM_EXTRA_RECURSIVE_TARGETS([style])
AC_SUBST(CPPSTYLE_CMD, 'find . -type f \( -iname "*.h" -or -iname "*.cpp" -or -iname "*.cc" -or -iname "*.tcc" \) -printf "Reformatting: %h/%f\n" -exec clang-format -i {} \;')