message: build: call AC_PATH_TOOL for dsymutil in macOS cross-compile
  While testing I noticed that we couldn't call dsymutil after LTO:
  ```bash
  ../libtool: line 10643: x86_64-apple-darwin16-dsymutil: command not found
  ```
  This updates configure to call `AC_PATH_TOOL` so that we end up with the
  full path to dsymutil, similar to `otool` and `install_name_tool`, ie:
  `/bitcoin/depends/x86_64-apple-darwin16/share/../native/bin/x86_64-apple-darwin16-otool`.
diff: @@ -650,6 +650,7 @@ case $host in
  BUILD_OS=darwin
  ;;
  *)
  + AC_PATH_TOOL([DSYMUTIL], [dsymutil], dsymutil)
  AC_PATH_TOOL([INSTALLNAMETOOL], [install_name_tool], install_name_tool)
  AC_PATH_TOOL([OTOOL], [otool], otool)
  AC_PATH_PROGS([GENISOIMAGE], [genisoimage mkisofs],genisoimage)
  