[CMake] getting a build parameter from a file

[Dave Yost]

First, my cmake code does this:
sets ${metals} from a cmake command-line argument
compares the contents of ${metals} against the contents of a file to note whether the variable is being changed since the last run of cmake
writes the new value to that file for next time
call an add_timestamp_file_target function. which touches the file metalsTimestamp if the variable is being changed or if the file doesn’t exist.
Then comes this:
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DMETALS=\"\\\"${metals}\\\"\"")
add_executable   (metals metals.cc <http://metals.cc/>)
add_dependencies (metals metalsTimestamp)

My add_timestamp_file_target function has to use one of
   add_custom_target
   add_custom_command
to make metalsTimestamp visible to the add_dependencies function.

Problem is, it seems that those commands have to touch the file every time cmake runs, but I want to touch it only when necessary.

Is there a way to make this work?

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