message: build: Add Boost.Process usage check
  Check that Boost.Process can be used without linking any libraries
  (header-only). Disable the functionality if that is not the case.
diff: @@ -1445,8 +1445,20 @@ if test "$use_external_signer" != "no"; then
  use_external_signer="no";
  ;;
  *)
  + AC_MSG_CHECKING([whether Boost.Process can be used])
  + AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <boost/process.hpp>]])],
  + [have_boost_process="yes"],
  + [have_boost_process="no"])
  + AC_MSG_RESULT([$have_boost_process])
  + if test "$have_boost_process" == "yes"; then
  use_external_signer="yes"
  AC_DEFINE([ENABLE_EXTERNAL_SIGNER], [1], [Define if external signer support is enabled])
  + else
  + if test "$use_external_signer" = "yes"; then
  + AC_MSG_ERROR([External signing is not supported for this Boost version])
  + fi
  + use_external_signer="no";
  + fi
  ;;
  esac
  fi
  