configure: add --enable-external-signer

This option replaces --with-boost-process

This prepares external signer support to be disabled by default.
It adds a configure option to enable this feature and to check
if Boost::Process is present.

This also exposes ENABLE_EXTERNAL_SIGNER to the test suite via test/config.ini

test/config.ini.in

@@ -23,3 +23,4 @@ RPCAUTH=@abs_top_srcdir@/share/rpcauth/rpcauth.py
 @BUILD_BITCOIND_TRUE@ENABLE_BITCOIND=true
 @ENABLE_FUZZ_TRUE@ENABLE_FUZZ=true
 @ENABLE_ZMQ_TRUE@ENABLE_ZMQ=true
+@ENABLE_EXTERNAL_SIGNER_TRUE@ENABLE_EXTERNAL_SIGNER=true