[libsigc++2/variadic_bind4] Remove C++ standard library checks.

[Murray Cumming <murrayc src gnome org>]

commit e438a231c25dfab4525a9ed37369771f1f54f1fd
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Mar 3 10:40:08 2016 +0100

    Remove C++ standard library checks.
    
    We do not actually use these, and it must have been years since
    any compiler didn't, for instance, have the std namespace.

 build/cxx_std.m4 |   69 ------------------------------------------------------
 configure.ac     |    2 -
 2 files changed, 0 insertions(+), 71 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index f8b24bb..d47944f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -50,8 +50,6 @@ MM_ARG_ENABLE_DOCUMENTATION
 MM_ARG_WITH_TAGFILE_DOC([libstdc++.tag], [mm-common-libstdc++])
 
 AC_LANG([C++])
-SIGC_CXX_HAS_NAMESPACE_STD
-SIGC_CXX_HAS_SUN_REVERSE_ITERATOR
 
 AS_IF([test "x$config_error" = xyes],
       [AC_MSG_FAILURE([[One or more of the required C++ compiler features is missing.]])])