Replace c++1y with c++14 as a choice for cpp_std

c++1y is deprecated in GCC 5 (see -std option in man page). Not sure about
Clang since the documentation is a bit more sparse. But it is probably a
good idea to keep them in sync and also to prefer c++14 over c++1y since
c++1y was only temporary until the standard was done.

compilers.py

@@ -1423,7 +1423,7 @@ class GnuCPPCompiler(CPPCompiler):
 
     def get_options(self):
         opts = {'cpp_std' : coredata.UserComboOption('cpp_std', 'C++ language standard to use',
-                                                     ['none', 'c++03', 'c++11', 'c++1y'],
+                                                     ['none', 'c++03', 'c++11', 'c++14'],
                                                      'c++11')}
         if self.gcc_type == GCC_MINGW:
             opts.update({
@@ -1469,7 +1469,7 @@ class ClangCPPCompiler(CPPCompiler):
 
     def get_options(self):
         return {'cpp_std' : coredata.UserComboOption('cpp_std', 'C++ language standard to use',
-                                                   ['none', 'c++03', 'c++11', 'c++1y'],
+                                                   ['none', 'c++03', 'c++11', 'c++14'],
                                                    'c++11')}
 
     def get_option_compile_args(self, options):