Setup libstdc++.so -> libgnustl_shared.so early

Setup libstdc++.so -> libgnustl_shared.so early

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.