Don't fail to set PARALLEL if PLATFORM is set manually.

Else the script ends up invoking `make -j--ndk-dir=/Users/thakis/src/insanity/ndk`,
due to PARALLEL not being set.

(Probably a good idea to liberally use `set -e`, and more generally to not use
bash scripts for anything longer than 15 lines.)
Merge pull request #2 from nico/platform

Don't fail to set PARALLEL if PLATFORM is set manually.