Fix C++11 check when --with-cxx=11 is used

AX_CXX_COMPILE_STDCXX() macro does not define HAVE_CXX11 when C++11 is
required (and not optional, as with --enable-cxx11), so the build was broken
when --with-cxx=11 was used under OS X as -stdlib=libc++ was not used.

Fix this and also leave only a single AX_CXX_COMPILE_STDCXX() check for C++11,
this saves us ~1000 lines in configure.

configure.in

@@ -1065,12 +1065,12 @@ if test -n "$wxWITH_CXX"; then
     dnl autoconf, not run, time.
     case "$wxWITH_CXX" in
         11)
+            AX_CXX_COMPILE_STDCXX(11,,optional)
             if test -n "$wxWITH_CXX_IS_OPTIONAL"; then
-                AX_CXX_COMPILE_STDCXX(11,,optional)
-            else
-                AX_CXX_COMPILE_STDCXX(11)
+                if test "$HAVE_CXX11" != 1; then
+                    AC_MSG_FAILURE([C++11 support was requested but is not available])
+                fi
             fi
-            have_cxxN=$HAVE_CXX11
             ;;
 
         14)
@@ -1078,6 +1078,10 @@ if test -n "$wxWITH_CXX"; then
             dnl and not --enable-cxx14 option.
             AX_CXX_COMPILE_STDCXX(14)
 
+            dnl Notice that because it's not optional, the macro doesn't define
+            dnl HAVE_CXX14. We don't use it anyhow so far, so it doesn't
+            dnl matter, but we could need to do it ourselves here in the future.
+
             dnl If we have C++14, we necessarily have C++11 too.
             HAVE_CXX11=1
             ;;