Move CMake's staging directory outside OUTPUT_DIR

[NiLuJe]

Otherwise, we're picking it up in the final packages

We've been shipping extra bits of jpeg-turbo & zsync2 because of that,
which explains why the tarball's size has suddenly ballooned...

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This change is 

[NiLuJe]

Randomly noticed this when testing a vanilla install from scratch the other day.

This got worse w/ zsync2 because of the two unstripped 2MB binaries (static C++ :/), but we've been shipping's jpeg-turbo's lib, bin & share folders for quite a while.

I hadn't noticed it on my end, because I'm using my own CMakeCross file, which already puts the staging directory in my own separate staging directory ;).

[NiLuJe]

Specifically:

Build_KOBO
├── bin
│   ├── cjpeg
│   ├── djpeg
│   ├── jpegtran
│   ├── rdjpgcom
│   ├── tjbench
│   ├── wrjpgcom
│   ├── zsync2
│   └── zsyncmake2
├── lib
│   └── static
│       └── libzsync2.a
└── share
    ├── doc
    │   └── libjpeg-turbo
    │       ├── example.txt
    │       ├── libjpeg.txt
    │       ├── LICENSE.md
    │       ├── README.ijg
    │       ├── README.md
    │       ├── structure.txt
    │       ├── tjexample.c
    │       ├── usage.txt
    │       └── wizard.txt
    └── man
        └── man1
            ├── cjpeg.1
            ├── djpeg.1
            ├── jpegtran.1
            ├── rdjpgcom.1
            └── wrjpgcom.1

8 directories, 23 files

5.5M    Build_KOBO

[Frenzie]

Whoops :-D