[build] Warn that only libconsensus can be built without boost

configure.ac

@@ -761,6 +761,9 @@ define(MINIMUM_REQUIRED_BOOST, 1.47.0)
 
 dnl Check for boost libs
 AX_BOOST_BASE([MINIMUM_REQUIRED_BOOST])
+if test x$want_boost = xno; then
+    AC_MSG_ERROR([[only libbitcoinconsensus can be built without boost]])
+fi
 AX_BOOST_SYSTEM
 AX_BOOST_FILESYSTEM
 AX_BOOST_PROGRAM_OPTIONS