[ISSUE] [RFC] set timestamps in a package only for files newer than commit date

[tornaria <tornaria@users.noreply.github.com>]

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New issue by tornaria on void-packages repository

https://github.com/void-linux/void-packages/issues/32638

Description:
The hook `pre-pkg/90-set-timestamps.sh` touches all files in a pkg to set the mtimes to the commit date. When a package installs files verbatim from its source, it might make sense to keep the original timestamp. This could be acomplished by using find predicate `-newermt` as in:
```diff
--- a/common/hooks/pre-pkg/90-set-timestamps.sh
+++ b/common/hooks/pre-pkg/90-set-timestamps.sh
@@ -5,6 +5,6 @@ hook() {
        # If SOURCE_DATE_EPOCH is set, set mtimes to that timestamp.
        if [ -n "$SOURCE_DATE_EPOCH" ]; then
                msg_normal "$pkgver: setting mtimes to %s\n" "$(date --date "@$SOURCE_DATE_EPOCH")"
-               find $PKGDESTDIR -print0 | xargs -0 touch -h --date "@$SOURCE_DATE_EPOCH"
+               find $PKGDESTDIR -newermt "@$SOURCE_DATE_EPOCH" -print0 | xargs -0 touch -h --date "@$SOURCE_DATE_EPOCH"
        fi
 }
```

I am thinking on a package like `pari-elldata` which ships only data files from its source. The alternative I found is to set `XBPS_USE_BUILD_MTIME=no` in the template, but (a) that is only good for this pkg because I know all the files in the package have a deterministic timestamp and (b) xlint complains when using this variable.

Rationale: the timestamp of the original files in the source is more canonical than the commit date.