Fix examples building with pkg-config being present on llvm-mingw

[amyspark]

By using CMake's official Find Modules and sidestepping pkg-config altogether, we're able to use imported targets without further complexity.

[farindk]

Thank you. By accident, I did much of the same work as in your patch just yesterday.
Could have saved me a lot of time had I seen this earlier :-)

I added the unset() lines from your patch (see above).
We cannot yet use the JPEG::JPEG syntax, because we still depend on an older cmake version for packaging on ancient Linux versions. Thus, I'll leave your pull request open as a reminder to use this syntax at a later time.

[amyspark]

Great to hear @farindk, thanks!