Add --destdir option to install.sh

Currently it seems like the `DESTDIR` option is ignored during the build process. This poses problems when packaging `rustc`, because it always tries to use `/` as a base path of the prefix. When packaging cargo for Arch Linux, everything should be installed into a separate directory, which content is used to create the package archive. At the moment this step requires `sudo`, which is discouraged in PKGBUILDs. Now, `make install DESTDIR=foo` prepends `DESTDIR` to all relevant paths during installation. 

This probably could be archived using `install --prefix=foo/usr/` as well, but the convention to separate prefix and destdir is rather common
Sure, done!