External dependencies required for build

[rpwoodbu]

In the process of building in a clean Crostini environment, I discovered that I needed to install the following packages:

• build-essential
• python
• gettext
• zip

build-essential is for GCC, in order to build tests on the local host. This was expected, although I didn't think it would block building without tests.
python is required for pnacl-clang, it would seem.
gettext provides envsubst, which is required for building //mosh_app:manifest.
zip is required to build the various uploadable packages.

These should either be included in the repo or pulled in by WORKSPACE. In either case, Bazel should be configured so it cannot see unspecified dependencies.

[rpwoodbu]

Getting rid of gettext (which has envsubst) is really the main one to contend with. All sane build environments should have the others. I'd prefer it to be more hermetic, but for a standalone project, that doesn't seem to be terribly realistic. (One can use the "docker" strategy in Bazel to build against a well-known environment, but that is very slow.)