Make the configure check for C++11 compiler mandatory

Remove stray "dnl", so that mandatory actually works with (my)
autoreconf.

configure.ac

@@ -105,7 +105,7 @@ AC_LANG([C++])
 PKG_PROG_PKG_CONFIG([0.20])
 
 # Check C++ compiler supports C++0x/C++11 feature
-AX_CXX_COMPILE_STDCXX_11([noext])
+AX_CXX_COMPILE_STDCXX_11([noext], [mandatory])
 
 # i686-w64-mingw32-g++ 4.6 does not support override keyword. For
 # those compilers, define CXX11_OVERRIDE to empty string. Otherwise

m4/ax_cxx_compile_stdcxx_11.m4

@@ -62,7 +62,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX_11], [dnl
   m4_if([$2], [], [ax_cxx_compile_cxx11_required=true],
         [$2], [mandatory], [ax_cxx_compile_cxx11_required=true],
         [$2], [optional], [ax_cxx_compile_cxx11_required=false],
-        [m4_fatal([invalid second argument `$2' to AX_CXX_COMPILE_STDCXX_11])])dnl
+        [m4_fatal([invalid second argument `$2' to AX_CXX_COMPILE_STDCXX_11])])
   AC_LANG_PUSH([C++])dnl
   ac_success=no
   AC_CACHE_CHECK(whether $CXX supports C++11 features by default,