fall back to system path for hotspot-perfparser

[GitMensch]

... if HOTSPOT_PERFPARSER not set and not found in libexec

this eases especially the use of static hotspot + hotspot-perfparser, as in that case those are the only two distributed elements

[fifbroman8]

Never mind. I will complie perfparser into hotspot. See https://github.com/fifbroman8/hotspot-bin/blob/main/demangle.patch

[GitMensch]

I don't think that perfparser should be a GUI application also not in your fork.
his initializes X11 and other display envs - which is bad if you do a hotspot --exportTo your.perfparser perf.data

Also this PR is not only relevanbt for your fork but intended to easily use the binary in other cases as well.

[GitMensch]

... also: I'd like to see a PR here that takes the "initial" demangler approach: using libiberty, when possible, and only if this isn't the case fallback to demangler plugins (not sure if the existing ones should be completely removed and libiberty made a hard dependency)

[milianw]

the string literal won't be deduped by the compiler, so please introduce a local. since you'll then have to touch this PR anyhow, please consider changing the code in the following way, which I subjectively find nicer:

// prefer configured one (returns an error in the caller if invalid)
if (const auto perfparser = qEnvironmentVariable("HOTSPOT_PERFPARSER"); !perfparser.isEmpty()) {
    return QStandardPaths::findExecutable(perfparser);
}

const auto hotspotPerfparser = QStringLiteral("hotspot-perfparser");

// common option: find binary by configured libexec
if (const auto parserBinary = Util::findLibexecBinary(hotspotPerfparser); !parserBinary.isEmpty()) {
    return parserBinary;
}

// fallback: try system paths
return QStandardPaths::findExecutable(hotspotPerfparser);

[fifbroman8]

I don't think that perfparser should be a GUI application also not in your fork. his initializes X11 and other display envs - which is bad if you do a hotspot --exportTo your.perfparser perf.data

Also this PR is not only relevanbt for your fork but intended to easily use the binary in other cases as well.

When linking into one executable, perfparser should have GUI because some kf6 modules has auto-initialization and will crash with QCoreApplication only.

[GitMensch]

When linking into one executable, perfparser should have GUI because some kf6 modules has auto-initialization and will crash with QCoreApplication only.

Then I highly suggest to keep both binaries separate - and with this PR which already has the "ok feature-wise" from Milian it will be even easier to have both directly next to each other.