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:
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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.