`make install` doesn't handle missing doc directory as a result of `--disable-docs`.

[wting]

$  ./configure --disable-docs --prefix=/opt/rust-nightly && make clean && make check && make -j4 && sudo make install
...
prepare: tmp/dist/rust-0.12.0-pre-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_back-*.so
prepare: tmp/dist/rust-0.12.0-pre-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libmorestack.a
prepare: tmp/dist/rust-0.12.0-pre-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler-rt.a
cp: cannot stat `doc': No such file or directory
make: *** [dist-install-dir-x86_64-unknown-linux-gnu-with-target-libs] Error 1

This fixes the problem: mkdir doc && sudo make install

[brson]

I believe this is fixed.

[wting]

Confirmed fixed.