Add back support for gmake 3.x

[imphil]

The "file" make function is only available in gmake 4.x, which isn't
available on RHEL/CentOS 7. Use alternative syntax to support older
gmake versions.

Fixes #2920

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[imphil]

The "touch" replacement isn't fully equivalent to this line -- did you intend to truncate the file (e.g. as a safety measure), or was it just to ensure that the file would always be present? (It should not be necessary to truncate it from what I could see, but it would be good to confirm.)

[wsnyder]

Good point, I think this does need to truncate, or each build will keep adding to the file (if an error occurs before it is removed).

[imphil]

Sounds good (and safer anyways), updated the PR with a version that does truncation.

[imphil]

OK, that doesn't work as-is, looking into it now.

[imphil]

Seems like CI is passing (and it is working on RHEL7), so let me explain the solution a bit more.

Two things which influenced this design:

• We want each dependency filename to be written to file in a separate shell call (otherwise we're back to square one, i.e. an overly long shell command line).
• Makefile variable expansion happens before the rule is evaluated.

The only way I found to have a single shell call per file to be echoed into the dependency file was to use $(shell echo ...) within the foreach loop. That happens at expansion time.

The tmp file needs to be truncated before that, so we cannot simply have a rule e.g.

echo "" > $@
$(foreach L, $(filter-out %.a,$^), $(shell echo $L >>$@.verilator_deplist.tmp))

because that would give us something like

# variable expansion
echo test.o >> my.verilator_deplist.tmp
echo test2.o >> my.verilator_deplist.tmp
# rule evaluation
echo "" > my.verilator_deplist.tmp

Having the rule and dependency on %.verilator_deplist.tmp ensures that the tmp file is truncated first.

The dependency on %.verilator_deplist.tmp is an ordering dependency (note the |), which doesn't appear in $^. (Otherwise the .tmp file would be part of the dependency list that we write to file.)

All that being said, I'm sure some Make-guru out there might have a better solution, but I (and my Googling) couldn't come up with a more beautiful alternative, unfortunately.

[wsnyder]

Thanks, I learned from that. I'm going to merge, thanks also for your work on this.