Finally, I could get past that error!

I noticed that I had this in my PKG_CONFIG_DIR: "/usr/lib/pkgconfig:/usr/lib64/pkgconfig:.."

While searching for a similar error message online, I came across this unrelated forum[1] which this comment caught my attention:

> The issue was that I included 32 bit glib2 in the of my *.pro file. I deleted those and retained only the 64 bit library paths, and it compiled without the above error.

So I removed "/usr/lib/pkgconfig" from PKG_CONFIG_DIR and that issue was gone, just like that.

Though, I need to make a note of that myself, and see if removing /usr/lib/pkgconfig from PKG_CONFIG_DIR breaks builds of anything else.

[1]: https://www.linuxquestions.org/questions/slackware-14/status_notifier_get_type-error-size-of-array-4175539700/