Recursive symlink_contents_to_dir is slow

In a similar vein to #804, even on Linux the recursive `symlink_contents_to_dir` process can be extremely slow. I'm not sure why it's necessary to recursively symlink dependencies rather than just the top-level dir.

When I compile a program that has a boost dependency using `rules_boost`, it spends nearly 9 minutes on this line in `build_script.sh`:
```
symlink_contents_to_dir $EXT_BUILD_ROOT/external/boost $EXT_BUILD_DEPS/include
```
Because rules_boost [adds its root dir to the include path](https://github.com/nelhage/rules_boost/blob/master/boost/boost.bzl#L88), it's not only symlinking headers but also every source/doc file. But why are we doing this instead of symlinking the top-level dependency directory? (i.e. what #805 proposes but on all platforms)

Start in build_script.sh at the symlink_contents_to_dir call for external/boost and compare it with the behavior discussed in #804 and #805. Determine whether recursive linking is required for this dependency layout; done means the recursive process no longer spends excessive time linking unnecessary source and documentation files.

Written by the indexing model from the issue text.