scons: Re-add -Werror for gem5 develop branch

This is removed from the stable branch to avoid build errors but should
included on the develop branch to aid developers.

This reverts commit 7dd61c8659.

Change-Id: I1fe249ce87aa8d70c1f092fc7db1554e6aee7355
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/67052
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>

SConstruct

@@ -420,6 +420,14 @@ for variant_path in variant_paths:
                     conf.CheckLinkFlag('-Wl,--threads')
                     conf.CheckLinkFlag(
                             '-Wl,--thread-count=%d' % GetOption('num_jobs'))
+
+        # Treat warnings as errors but white list some warnings that we
+        # want to allow (e.g., deprecation warnings).
+        env.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.",