message: build: remove check for Boost Process header
  Now that we require Boost 1.64.0+, Boost Process will be available.
diff: @@ -1432,18 +1432,6 @@ if test x$use_boost = xyes; then
  AX_BOOST_SYSTEM
  AX_BOOST_FILESYSTEM
  - dnl Opt-in to Boost Process if external signer support is requested
  - if test "x$use_external_signer" != xno; then
  - AC_MSG_CHECKING(for Boost Process)
  - AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <boost/process.hpp>]],
  - [[ boost::process::child* child = new boost::process::child; delete child; ]])],
  - [ AC_MSG_RESULT(yes)
  - AC_DEFINE([ENABLE_EXTERNAL_SIGNER],,[define if external signer support is enabled])
  - ],
  - [ AC_MSG_ERROR([Boost::Process is required for external signer support, but not available!])]
  - )
  - fi
  -
  if test x$suppress_external_warnings != xno; then
  BOOST_CPPFLAGS=SUPPRESS_WARNINGS($BOOST_CPPFLAGS)
  fi
  @@ -1451,6 +1439,9 @@ if test x$use_boost = xyes; then
  BOOST_LIBS="$BOOST_LDFLAGS $BOOST_SYSTEM_LIB $BOOST_FILESYSTEM_LIB"
  fi
  +if test "x$use_external_signer" != xno; then
  + AC_DEFINE([ENABLE_EXTERNAL_SIGNER],,[define if external signer support is enabled])
  +fi
  AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER], [test "x$use_external_signer" = "xyes"])
  dnl Check for reduced exports
  