SCons: restore Werror option in src/SConscript
SCons: restore Werror option in src/SConscript

Partial backout of cset 8b223e308b08.

Although it's great that there's currently no need
for Werror=false in the current tree, some of us
have uncommitted code that still needs this option.

pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,vovojh/gem5,vovojh/gem5,hoangt/tpzsimul.gem5,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-