Add -fno-strict-aliasing workaround for Linux GCC targets

Add -fno-strict-aliasing workaround for Linux GCC targets

Most Linux distros work around aliasing problems in Mesa by compiling
with the GCC option -fno-strict-aliasing. Two examples:

  https://bugs.freedesktop.org/show_bug.cgi?id=6046
  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=394311

This makes -fno-strict-aliasing the default with a comment that
developers should consider commenting it out. There is a already a note
about these bugs in docs/helpwanted.html.