scons: Re-add -Werror for the gem5 develop branch

This reverts commit 3c61200c2b,
https://gem5-review.googlesource.com/c/public/gem5/+/48584

Change-Id: Idc071168857746f4c8d3e577949939ecf1761cda
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48812
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -330,6 +330,12 @@ if main['GCC'] or main['CLANG']:
     if GetOption('gold_linker'):
         main.Append(LINKFLAGS='-fuse-ld=gold')
 
+    # Treat warnings as errors but white list some warnings that we
+    # want to allow (e.g., deprecation warnings).
+    main.Append(CCFLAGS=['-Werror',
+                         '-Wno-error=deprecated-declarations',
+                         '-Wno-error=deprecated',
+                        ])
 else:
     error('\n'.join((
           "Don't know what compiler options to use for your compiler.",