CMake fix passing multiple paths to libdjinterop

[mr-smidge]

CMake does not appear to pass lists of paths correctly to external projects when invoking similarly to the below, affecting builds involving libdjinterop with sources fetched from GitHub:

ExternalProject_Add( ...
  -DCMAKE_PREFIX_PATH:PATH={CMAKE_PREFIX_PATH}
  ...)

The above causes path separators to be replaced with whitespace. This breaks the Windows build when run from Visual Studio, where the path contains multiple elements as follows:

• C:<path-to-buildenv>\mixxx-dependencies-2.3-x64-windows-3d70970e752542db9901e4aa2656f215699dc102
• C:<path-to-buildenv>\mixxx-dependencies-2.3-x64-windows-3d70970e752542db9901e4aa2656f215699dc102\installed\x64-windows
• C:<path-to-buildenv>\mixxx-dependencies-2.3-x64-windows-3d70970e752542db9901e4aa2656f215699dc102\installed\x64-windows\debug

When these paths are set in CMAKE_PREFIX_PATH with semicolon delimiter, it is transformed to whitespace upon invoking the external project, meaning only the first path gets searched. Because the dependencies of libdjinterop can only be found in the second/third paths, this leads to a missing dependency error when trying to build libdjinterop.

A workaround (inspired by this SO post) is to delimit the paths with something other than semicolon, which is what this PR does.

[JoergAtGithub]

Hello Adam,
I just compiled this PR with VS2019 on Win7 the build suceeded and I see the menu Export to Engine Prime now.
Thank you!
Jörg

[Holzhaus]

LGTM, thank you.