druntime-test: Detect musl more robustly

[the-horo]

Try to detect musl on more systems than just Alpine.

This also fixes an issue in the Github runners where the default shell is dash which doesn't understand the &> operator leading to the musl-excluded tests to also be skipped on glibc linux.

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[the-horo]

Spotted this in the ldc CI: ldc-developers/ldc#4969 (comment)

[the-horo]

One issue with this PR is that it now treats IS_MUSL=0 as musl being present, which I need to fix.

[kinke]

That's IMO some weird/unintuitive syntax; found it in https://www.gnu.org/software/make/manual/html_node/Setting.html. I think the $(shell …) wrapper is easier to understand.

[the-horo]

The reason for using != is that it's part of posix, not a gnu extension. It's also documented on https://www.gnu.org/software/make/manual/html_node/Shell-Function.html.

I don't object to changing it, if the gnu syntax is preferred, since the makefiles already rely on it

[kinke]

Oh, so that != is actually standard, whereas $(shell) is an extension?! - I count 3 != occurrences in DMD master Makefiles (all Valgrind-related), and 16 $(shell).

No too strong opinion from my side, except that I find the != extremely unintuitive.

[kinke]

One issue with this PR is that it now treats IS_MUSL=0 as musl being present, which I need to fix.

No strong opinion from my side, we can also go with defined/undefined instead of checking for a 1 - the syntax with ifdef/ifndef seems a bit nicer to read to me (i.e., what you have right now).

[kinke]

Oh and please document the new variable in common.mak.

[the-horo]

One issue with this PR is that it now treats IS_MUSL=0 as musl being present, which I need to fix.

No strong opinion from my side, we can also go with defined/undefined instead of checking for a 1 - the syntax with ifdef/ifndef seems a bit nicer to read to me (i.e., what you have right now).

My only concern is changing something that worked before. However, dmd and ldc should be the only ones that touch these files so, if you're fine with it, I'll keep the syntax.