Specify a C++ standard to build with (C++11)

Without being specified, we have c++98 on gcc4
and c++11 on newer gccs.
Eventually c++14 will be the default.

Better to stick with a specific standard, thus being predictable,
than trying to write the code in a way that builds with all of them.

Signed-off-by: falkTX <falktx@gmail.com>

wscript

@@ -217,6 +217,7 @@ def configure(conf):
         conf.env.append_unique('CXXDEFINES', '_POSIX')
 
     conf.env.append_unique('CXXFLAGS', '-Wall')
+    conf.env.append_unique('CXXFLAGS', '-std=gnu++11')
     conf.env.append_unique('CFLAGS', '-Wall')
 
     if conf.env['IS_MACOSX']: