build: Add a top-level forwarding target for src/* objects
build: Add a top-level forwarding target for src/* objects

Fixes #3955. It's hackish, but seems to always function as expected. Examples:

make src/bitcoind
make src/qt/bitcoin-qt
make src/libbitcoin.a

Rebased-from: 77a055d04979dc2b3ec4064e99c69259a4216f12 #4322

GroundRod/anoncoin,GroundRod/anoncoin,Anoncoin/anoncoin,GroundRod/anoncoin,GroundRod/anoncoin,Anoncoin/anoncoin,Anoncoin/anoncoin,Anoncoin/anoncoin,Anoncoin/anoncoin,GroundRod/anoncoin