scons: Switch to c++17.

Change-Id: I90d121ab4dbf53f3bd30bcdb5f5101d9c7cdc437
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45902
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -316,8 +316,8 @@ if main['GCC'] or main['CLANG']:
     main.Append(CCFLAGS=['-Wall', '-Wundef', '-Wextra',
                          '-Wno-sign-compare', '-Wno-unused-parameter'])
 
-    # We always compile using C++14
-    main.Append(CXXFLAGS=['-std=c++14'])
+    # We always compile using C++17
+    main.Append(CXXFLAGS=['-std=c++17'])
 
     if sys.platform.startswith('freebsd'):
         main.Append(CCFLAGS=['-I/usr/local/include'])