Replace gcc with clang

Replace gcc with clang

Only clang has official support for Android, and trying to maintain
a working gcc will only get messier over time.

Remove gcc and g++ packages, and setup gcc and g++ as symlinks to
clang.

Let clang replace gcc, but note that 'apt dist-upgrade' needs to
be run once if a user has both clang and gcc installed.

Fixes #369.