scons: Adjust the version of C++ to C++14.

Change-Id: I318d337fc61bca0ae40413c23ee36d59d45a79bc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34820
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -317,8 +317,8 @@ if main['GCC'] or main['CLANG']:
     # we consistently violate
     main.Append(CCFLAGS=['-Wall', '-Wundef', '-Wextra',
                          '-Wno-sign-compare', '-Wno-unused-parameter'])
-    # We always compile using C++11
-    main.Append(CXXFLAGS=['-std=c++11'])
+    # We always compile using C++14
+    main.Append(CXXFLAGS=['-std=c++14'])
     if sys.platform.startswith('freebsd'):
         main.Append(CCFLAGS=['-I/usr/local/include'])
         main.Append(CXXFLAGS=['-I/usr/local/include'])