Localized documentation doesn't support out-of-tree builds

[huonw]

$ make docs-l10n
cfg: build triple i686-unknown-linux-gnu
cfg: host triples i686-unknown-linux-gnu
cfg: target triples i686-unknown-linux-gnu
cfg: enabling more debugging (CFG_ENABLE_DEBUG)
cfg: host for i686-unknown-linux-gnu is i386
cfg: os for i686-unknown-linux-gnu is unknown-linux-gnu
cfg: using gcc
cfg: no llnextgen found, omitting grammar-verification
make: *** No rule to make target `doc/l10n/ja/tutorial.md', needed by `doc/l10n/ja/tutorial.html'.  Stop.

Fixing that immediate issue results in the po4a configuration being incorrect:

/home/huon/rust/src/doc/po4a.conf:8: The 'src/doc/complement-bugreport.md' master file does not exist.

which is almost certainly because I'm doing an out of tree build (specifically, I've checked out rust to ~/rust and configured in/am building in ~/rust/build).

[steveklabnik]

With #19709 , this now builds for me, but I didn't try out of tree.

[steveklabnik]

Localized docs are now gone, so this is irrelevant.