Building against system's versions of tarball dependencies

[OPNA2608]

I would prefer to build this repository devendored, supplying my system's versions of Boost, cc65, RtMidi etc instead of downloading the nesicide-deps tarball. Would it be possible to add qmake config options or some detection mechanism to use system-installed versions of them instead? Getting rid of the deps tarball altogether would be even better, but baby steps.

Example from another project:

https://github.com/rerrahkr/BambooTracker/blob/fdb048e67c7fdc18b709b55fee5300cf90a611fb/BambooTracker/BambooTracker.pro#L485-L510
https://github.com/rerrahkr/BambooTracker/blob/fdb048e67c7fdc18b709b55fee5300cf90a611fb/submodules/RtMidi/RtMidi.pro#L3-L10

If CONFIG+=system_rtmidi is passed to qmake, it will use pkg-config to find the system's RtMidi installation and error out of it's missing. If it isn't passed, it's expected that the user cloned the repo with --recurse-submodules to get the source code from the https://github.com/thestk/rtmidi/ repository and RtMidi is built from source & linked statically into the final binary.