Fix MSYS2 compatibility

[amyspark]

Hey @mm2,

This PR groups together the fixes needed to enable CI for all Intel MSYS2 flavors:

• used concurrency groups to supersede builds quickly when force-pushing
• fixed the missing exported APIs issue I reported in the mialing list
• fixed your usage of timespec_get in the threaded plugin test bench, which on Windows, is UCRT-only and C11 or higher

As regards the missing private APIs-- given that you intend to use GCC's visibility to export the functions in Windows, do you want me to remove the vs_module_defs in that case?

[amyspark]

@dragonmux @kmilos Seems that newer setup-msys2 takes care of both of these, so I've moved forward with the requested changes.

[mm2]

@amyspark ok, I was done exactly that, you sent the PR before I could commit.