cmake: Fix malformed CMake package location on Windows

[PatTheMav]

Description

Fix package export destination for Windows to meet CMake requirements exclusive to that platform.

Motivation and Context

CMake doesn't expect CMake package files to be separated in subdirectories on Windows and instead expects all files to be put in a single directory cmake found within one of the PREFIX paths.

Also fixes circular dependency in w32-pthreads CMake package config file.

Required to unblock obsproject/obs-plugintemplate#126.

How Has This Been Tested?

Tested on Windows 11.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

What should be the observed before/after on this?

[PatTheMav]

What should be the observed before/after on this?

Without this fix it's impossible to build/link anything on Windows that uses libobs as the dependency on w32-pthreads creates a circular dependency on libobs again.

As the plugin template has used the legacy path so far, the issue wasn't exposed and apparently no-one else attempted to use a non-legacy libobs build procedure to link against it.

[RytoEX]

And the package change location change?

[PatTheMav]

And the package change location change?

Before: <prefix>/cmake/<target>/<target>Config.cmake

After: <prefix>/cmake/<target>Config.cmake,
which hits the second variant according to https://cmake.org/cmake/help/latest/command/find_package.html#id10.

Windows-specific alternatives would have been:

• <prefix>/cmake/<target>/cmake/<target>Config.cmake
• <prefix>/<target>/<target>Config.cmake
• <prefix>/<target>/cmake/<target>/<target>Config.cmake
• <prefix>/<target>/lib/cmake/<target>/<target>Config.cmake
• <prefix>/<target>/lib/<target>/<target>Config.cmake
• <prefix>/<target>/lib/<target>/cmake/<target>Config.cmake

Variant #2 seemed to be the simplest one to meet Windows-specific requirements and should be in line with how the legacy system exported the files.