Tweak configure flags.

When -debug, --disable-llvm-assertions is added to reduce compile time
and improve performance. Otherwise, disable various optimizations to
make debugging easy.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #11 from omasanori/configure-flags

Tweak configure flags.
Update the rust ebuild

 - Port to git-r3 and utilize submodules (allows avoiding refetching)
 - Allow building using local rust instead of prebuilt one
 - Drop dependency on llvm which is not needed and might cause conflicts
   during bootstrapping
Merge pull request #13 from uzytkownik/master

Update the rust ebuild