build: libtoolize bitcoin

Minimal changes to make it possible to build shared libraries from Bitcoin Core:

    libbitcoincore_server.so.1
    libbitcoincore_wallet.so.1
    libbitcoincore_cli.so.1
    libbitcoincore_common.so.1

Libtool automagically adds --enable-shared and --enable-static options.

This is a step towards being able to split off the wallet or in general
to make it possible to develop other Bitcoin-based stuff outside the core repository.
It is a minimal change contained to the build system. The current
division into libraries may not be ideal for external usage, this can be
changed later.

Default behavior should be unchanged: building shared libraries is
disabled by default.