For some reason Ubuntu Focal repositories now have `llvm-13` virtual
package which can't be installed, but successfully fools our check,
resulting in no clang/llvm being installed...
```
$ apt show llvm-13
Package: llvm-13
State: not a real package (virtual)
N: Can't select candidate version from package llvm-13 as it has no candidate
N: Can't select versions from package 'llvm-13' as it is purely virtual
N: No packages found
$ apt install --dry-run llvm-13
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package llvm-13 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'llvm-13' has no installation candidate
```
to make it easier to figure out why it fails.
For example in https://github.com/systemd/systemd/runs/4799774735?check_suite_focus=true
it failed with
```
meson.build:1003:8: ERROR: Command "/usr/bin/clang -print-targets" failed with status 1.
A full log can be found at /home/runner/work/systemd/systemd/build/meson-logs/meson-log.txt
Error: Process completed with exit code 1.
```
and it wasn't clear what exactly happened there.
Let's assign a specific -Dcryptolib= value to each job to have at least
some coverage for all supported cryptolibs without unnecessarily
multiplying the test matrix.
Should provide coverage for #21880.
ATTOW llvm-11 got into focal-updates, which conflicts with llvm-11
provided by the apt.llvm.org repositories. Let's use the system
llvm package if available in such cases to avoid that.
It seems there is another meson (0.57.0) regression preventing clang from
building systemd with --optimization=3 -Db_lto=true
By analogy with https://github.com/google/oss-fuzz/pull/5199 let's just switch
to 0.56.2 for the time being
Frantisek Sumsal
Evgeny Vereshchagin
Jan Janssen
наб
Zbigniew Jędrzejewski-Szmek