Fixed invalid CMakeSettings.json generation

[qgazq]

I noticed that CMakeSettings.json is currently being generated with an invalid key when windows_buildenv.bat is used.
Visual Studio 19 therefore won't load it, and gives:
Failed to parse CMakeSettings.json. Bad JSON escape sequence: \=. Path 'configurations[0].variables[5].value', line 40, position 22.
The problem seems to be CMAKE_PREFIX_PATH is no longer setup in the bat file,
So the line: CALL :AddCMakeVar2CMakeSettings_JSON "CMAKE_PREFIX_PATH" "STRING" "!CMAKE_PREFIX_PATH:\=\\!"
Causes a line "value": "\=\\", in the CMakeSettings.json which isn't valid.
My fix is to add a set back into the file matching one that was removed by an earlier commit, I'm not sure if its the best solution as I'm not sure what if anything CMAKE_PREFIX_PATH is used for. It's possible we may not need it and it can be removed?

[Be-ing]

@JoergAtGithub can you review this?

[JoergAtGithub]

I just tested this. I can confirm that Main is broken here. I didn't notice it, because the last CMAKE_PREFIX path remains set system wide, until you overwrite it. In my case, there were multiple old CMAKE_PREFIX paths concatenated. And one of them pointed to the correct files.

With this patch always the correct CMAKE_PREFIX path is set. This PR works for me as well for VisualStudio GUI builds, as for command line builds!
Thank you for fixing this!

[Holzhaus]

Alright, then lets merge this. Thanks to both of you!

[daschuer]

Hi @qgazq, thank you for your contribution.
It looks like we have missed to ask for your permission to distribute your code before merge. Can you catch that up, sign https://docs.google.com/a/mixxx.org/spreadsheet/viewform?formkey=dEpYN2NkVEFnWWQzbkFfM0ZYYUZ5X2c6MQ and comment here when done? I will then put your name to the Mixxx 2.4 contributor list in the Mixxx about box.

[qgazq]

Hi. It wasn't much of a contribution, I was just getting it to build so I could debug an issue I was having, I'm not sure it's worth a mention on the about list. But happy to help. I've filled out the form for the record, I wasn't sure if you needed my real name, so thats what I've put on the form (its also on my github profile).

[daschuer]

Finding an issue can take a lot of time even if the resulting fix is only one line and than do the extra effort and contribute a PR makes the difference. This deserves to be mentioned.