Get rid of relying on pre-set env variables in build scripts

[emaxx-google]

Currently, one has to run "source env/activate" before being able to compile the code, otherwise the Make will fail with unclear errors (like complaining about the NACL_SDK_ROOT directory).

We should probably get rid of that and change our Makefile's to do this automatically, since the current situation is confusing for developers. Historically, this was done in accordance to the standard Native Client development practice, allowing to reuse an SDK between multiple projects or switch between different SDK versions, but in the light of NaCl deprecation this isn't useful anymore.