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

src/util/system.cpp

@@ -5,9 +5,9 @@
 
 #include <util/system.h>
 
-#ifdef HAVE_BOOST_PROCESS
+#ifdef ENABLE_EXTERNAL_SIGNER
 #include <boost/process.hpp>
-#endif // HAVE_BOOST_PROCESS
+#endif // ENABLE_EXTERNAL_SIGNER
 
 #include <chainparamsbase.h>
 #include <sync.h>
@@ -1247,7 +1247,7 @@ void runCommand(const std::string& strCommand)
 }
 #endif
 
-#ifdef HAVE_BOOST_PROCESS
+#ifdef ENABLE_EXTERNAL_SIGNER
 UniValue RunCommandParseJSON(const std::string& str_command, const std::string& str_std_in)
 {
     namespace bp = boost::process;
@@ -1282,7 +1282,7 @@ UniValue RunCommandParseJSON(const std::string& str_command, const std::string&
 
     return result_json;
 }
-#endif // HAVE_BOOST_PROCESS
+#endif // ENABLE_EXTERNAL_SIGNER
 
 void SetupEnvironment()
 {