build: multiprocess autotools changes

autoconf and automake changes to support multiprocess gui/node/wallet execution.

This adds a new --enable-multiprocess flag, and build configuration code to
detect libraries needed for multiprocess support. The --enable-multiprocess
flag builds new bitcoin-node and bitcoin-gui executables, which are updated in
https://github.com/bitcoin/bitcoin/pull/10102 to communicate across processes.
But for now they are functionally equivalent to existing bitcoind and
bitcoin-qt executables.

.gitignore

@@ -5,6 +5,8 @@
 src/bitcoin
 src/bitcoind
 src/bitcoin-cli
+src/bitcoin-gui
+src/bitcoin-node
 src/bitcoin-tx
 src/bitcoin-wallet
 src/test/fuzz