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.
This fixes Android 6 problems (so closes#26).
It also rebuilds termux-elf-cleaner when needed
and bumps some build revisions for packages
needed updates to remove DT_VERDEF and DT_VERDEFNUM.
