Added switches for gcc-4.0. "-fstrict-aliasing" is now added only for

Added switches for gcc-4.0. "-fstrict-aliasing" is now added only for
gcc 3.0 since 3.3 and newer turn in by default with -O2 or higher.
Added CWARNS_OFF, which turns off even the warnings which default to on.

Expect lots of warnings with gcc 4.0.0; some of them are caused by
compiler bugs (e.g., http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21800).