build: libtoolize bitcoin core

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

• libbitcoincore_server.so.1
• libbitcoincore_wallet.so.1 (if --enable-wallet)
• libbitcoincore_cli.so.1
• libbitcoincore_common.so.1

Libtool automagically adds --enable-shared and --enable-static options. Bitcoind and the other executables link against the shared libraries if available.

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 improved later.

Default behaviour should be unchanged: building shared libraries is disabled by default. One step in implementing #3882 (closed) .