variable expansion at command section on GNUMake

* When 'variable expansion' at Command Section, is processed at GNUMake
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In GNUMake source code, command line section of each Target seems to be expanded at 'new_job()' function.
In detail, 'for-loop part' under '/* Expand the command lines and store the results in LINES.  */' comment.

And then, first command line is fetched at 'job_next_command()' function.
Then, when 'job slot(in case of using mutlple job)' is available, this command line that expansion is done, is executed on newly created child process.

new_job
    - expand command line variables (line-by-line)
    - waiting  until job slot is available.
    - job is executed.. 

Note that all variables in command section, are expanded before processing first command line.
Note that, variables in pre-requisites and dependencies are expanded before expanding command line section.

So, following gnumake code is dangerous.

out/target-file: <...>
    ./generate-target-file.sh $@
    ./postprocess.sh $(shell readlink -e $@)
    
[ Issue ]
"$(shell ...) function" part is expanded before "./generate-target-file.sh" is executed.
So, af the first build, 'out/target-file' doesn't exist, "readlink -e $@" return empty string. And this may lead to unexpected result.

--- should be modified like below
    
out/target-file: <...>
    ./generate-target-file.sh $@
    ./postprocess.sh $$(readlink -e $@)