message: build: Fix Boost.Process check for Boost 1.73 and older
diff: @@ -1446,9 +1446,13 @@ if test "$use_external_signer" != "no"; then
  ;;
  *)
  AC_MSG_CHECKING([whether Boost.Process can be used])
  + TEMP_LDFLAGS="$LDFLAGS"
  + dnl Boost 1.73 and older require the following workaround.
  + LDFLAGS="$LDFLAGS $PTHREAD_CFLAGS"
  AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <boost/process.hpp>]])],
  [have_boost_process="yes"],
  [have_boost_process="no"])
  + LDFLAGS="$TEMP_LDFLAGS"
  AC_MSG_RESULT([$have_boost_process])
  if test "$have_boost_process" = "yes"; then
  use_external_signer="yes"
  