add_custom_target(Provider COMMAND some-generator -o side-effect)
add_custom_command(
  OUTPUT explicit-output
  COMMAND cp side-effect explicit-output
  DEPENDS side-effect
  )
add_custom_target(Consumer DEPENDS explicit-output)
add_dependencies(Consumer Provider) # ensure side-effect is up to date

include(ExternalProject)
ExternalProject_Add(OtherProj ...)
add_library(OtherProj::OtherLib STATIC IMPORTED)
set_target_properties(OtherProj::OtherLib PROPERTIES ...)
add_dependencies(OtherProj::OtherLib OtherProj) # ensure OtherProj is up to date
add_executable(MyExe ...)
target_link_libraries(MyExe OtherProj::OtherLib) # gets dependency on OtherProj through imported target

rule R
    command = $COMMAND
build always: R
    COMMAND = echo 1 >> out
build out: phony out
build out-copy: R out || always
    COMMAND = cp out out-copy
build all: phony out-copy always
default all

rule R
    command = $COMMAND
    restat = 1
build always out: R
    COMMAND = echo 1 >> out
build out-copy: R out || always
    COMMAND = cp out out-copy
build all: phony out-copy always
default all