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.
Let build-package.sh setup the $PREFIX/lib/libstdc++.so symlink
before building any package, to make sure that all C++ applications
link against a fully featured C++ standard library.
Avoid explicitly declaring a dependency on the libgnustl package
since everyone may link against it at will and instead mark it as
essential.