CMake: Ensure we use "-pthread" instead of "-lpthread" for thread support

[tamiko]

We will need this in 9.4 as well.

[tamiko]

/rebuild

[tamiko]

The difference for gcc-9.4.0 (on Ubuntu 20.04.1) is subtle: -lpthread results in a final linkage via collect2 [...] -lpthread -lstdc++ [...] whereas -pthread results in collect2 [...] -lstdc++ -lpthread [...] (and some different options for an lto plugin). The former unfortunately leads to linker errors if we happen to use threading facilities from the standard libraries in the example step directly.

[tjhei]

Awesome, thank you! Would you mind adding the DSO error message to the comments somewhere.

[tamiko]

@tjhei done.