Fix STB detection when STB is manually specified

[rcurtin]

I discovered while debugging #3641 that CMake was not actually configuring mlpack as though STB was available, even though STB's location was being specified in the CMake call! Digging into this, I found that our CMake code was using the STB_IMAGE_FOUND variable, but the script CMake/FindStbImage.cmake doesn't always set that variable.

The correct variable to use is actually StbImage_FOUND, which is set by the find_package_handle_standard_args() CMake function. So, I adapted our CMake configuration to use that instead, and removed the manually-set STB_IMAGE_FOUND variable, which actually will not be set when a user manually specifies the STB include directory.

[conradsnicta]

Sidenote. It may be worthwhile to consider removing STB integration from mlpack, in order to reduce the maintenance burden. Having a dependency is an invitation for that dependency (and the associated handling of that dependency) to break things. It can be argued that mlpack's primary domain is machine learning on user supplied data, and handling of non-trivial image formats (eg. jpeg, png) should be handled by the user before passing the data to mlpack.

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍