build: disable -fstrict-aliasing if gcc < 4.6.0

A compiler bug in older versions of gcc makes it do unsafe optimizations at -O1
and higher. This manifested itself with (at least) gcc 4.5.2 on SmartOS because
it made V8 hang in a busy loop.

See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45883