Escaping dollar sign for subshell in custom_command

observed in cmake version 3.5.2 (compiled from source on a Linux Ubuntu (legacy installation)).

When using the posix construct $(instruction) (actually attempting the make-escape instruction $$(instruction) substitute with the output of a subshell (equivalent but easier to nest than backticks (instruction)) in a custom command, The $$( is expanded to \$$\$$( in VERBATIM modus, and to $$ ( in non-VERBATIM modus.

A similar issue also arrises with $${ - } (makefile escaped to shell variable expansion).

e.g.

This instruction copies a directory to another host (using tar|ssh tar), then executes make on the remote host, to transfer the result - on success - back to the cmake-running host.

add_custom_target(TARGET ${module}-other
    COMMAND tar -cf - | ssh otherhost (T="$(mktemp -d)" && tar -C "$${T}" -xf - && make -C "$${T}" && tar -C "$${T}" -cf - result* | tar -xf -
)

The example is simple an can easily be converted to a working solution:

add_custom_target(TARGET ${module}-other
    COMMAND tar -cf - | ssh otherhost (T=`$(mktemp -d)` && tar -C "$$T" -xf - && make -C "$$T" && tar -C "$$T" -cf - result* | tar -xf -
)

However what about this slight twist:

add_custom_target(TARGET ${module}-other
    COMMAND tar -cf - | ssh otherhost (T="$(mktemp -d -p "$(custom_dir_outputting)" )" && tar -C "$${T}" -xf - && make -C "$${T}" && tar -C "$${T}" -cf - result* | tar -xf -
)