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This issue caused MySQL plugin headaches for me: I installed the prebuilt Mac 4.8.4 package, which have a build key of macosx macx-cocoa g++-4 full-config. I then downloaded the source to compile just the MySQL plugin. On 10.8.2, pkg-config isn't there. So while the plugin built and installed fine, its build key was set to macosx macx-cocoa g++-4 no-pkg-config, and the plugin refused to load.
I'm not well versed in pkg-config, but I'm at a loss as to how it could possibly affect plugin compatibility, so I agree with the reporter.
FWIW, I figured I could either recompile/install all of Qt, or edit the generated src/corelib/global/qconfig.h to force it to report full-config instead of no-pkg-config for QT_BUILD_KEY. I chose the quick and hacky-er second choice after failing to find a way to force configure to do it for me.