OCIO 2.6.0 FTBFS with MinGW

Start with src/libutils/oglapphelpers/dxapp.cpp at the failing _uuidof calls and inspect the WIN32 configuration around line 291 of CMakeLists.txt. Reproduce the MinGW build with DirectX enabled and trace how the toolchain is selected. Done means OCIO 2.6.0 builds successfully on MinGW without the reported dxapp.cpp errors.

Newly released OCIO 2.6.0 FTBFS on MinGW because it enables DirectX OOTB:

FAILED: [code=1] src/libutils/oglapphelpers/CMakeFiles/oglapphelpers.dir/dxapp.cpp.obj 
  D:\M\msys64\ucrt64\bin\g++.exe -DOCIO_DIRECTX_ENABLED -DOCIO_GL_ENABLED -ID:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers -ID:/_/B/src/OpenColorIO-2.6.0/include/OpenColorIO/.. -ID:/_/B/src/OpenColorIO-2.6.0/include/OpenColorIO -ID:/_/B/src/OpenColorIO-2.6.0/src/OpenColorIO -ID:/_/B/src/build-UCRT64/include/OpenColorIO -ID:/_/B/src/build-UCRT64/src/OpenColorIO -isystem D:/_/B/src/build-UCRT64/ext/build/DirectX-Headers/directx-headers-src/include -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -Wp,-D__USE_MINGW_ANSI_STDIO=1 -O3 -DNDEBUG -std=c++17 -fvisibility=hidden -fno-keep-inline-dllexport -DUSE_GCC -Wall -Wextra -Wswitch-enum -MD -MT src/libutils/oglapphelpers/CMakeFiles/oglapphelpers.dir/dxapp.cpp.obj -MF src\libutils\oglapphelpers\CMakeFiles\oglapphelpers.dir\dxapp.cpp.obj.d -o src/libutils/oglapphelpers/CMakeFiles/oglapphelpers.dir/dxapp.cpp.obj -c D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp
  In file included from D:/M/msys64/ucrt64/include/winbase.h:2864,
                   from D:/M/msys64/ucrt64/include/windows.h:70,
                   from D:/M/msys64/ucrt64/include/rpc.h:16,
                   from D:/_/B/src/build-UCRT64/ext/build/DirectX-Headers/directx-headers-src/include/directx/d3d12.h:26,
                   from D:/_/B/src/build-UCRT64/ext/build/DirectX-Headers/directx-headers-src/include/directx/d3dx12.h:11,
                   from D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/hlsl.h:15,
                   from D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.h:10,
                   from D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp:10:
  D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp: In function 'void OpenColorIO_v2_6::{anonymous}::GetHardwareAdapter(IDXGIFactory1*, IDXGIAdapter1**, bool)':
  D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp:59:112: error: expected primary-expression before ')' token
     59 |                     if (SUCCEEDED(D3D12CreateDevice(adapter.Get(), D3D_FEATURE_LEVEL_11_0, _uuidof(ID3D12Device), nullptr)))
        |                                                                                                                ^
  D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp:59:92: error: '_uuidof' was not declared in this scope
     59 |                     if (SUCCEEDED(D3D12CreateDevice(adapter.Get(), D3D_FEATURE_LEVEL_11_0, _uuidof(ID3D12Device), nullptr)))
        |                                                                                            ^~~~~~~
  D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp:78:112: error: expected primary-expression before ')' token
     78 |                     if (SUCCEEDED(D3D12CreateDevice(adapter.Get(), D3D_FEATURE_LEVEL_11_0, _uuidof(ID3D12Device), nullptr)))
        |                                                                                                                ^
  D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp:78:92: error: '_uuidof' was not declared in this scope
     78 |                     if (SUCCEEDED(D3D12CreateDevice(adapter.Get(), D3D_FEATURE_LEVEL_11_0, _uuidof(ID3D12Device), nullptr)))
        |                                                                                            ^~~~~~~
  D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp: In constructor 'OpenColorIO_v2_6::DxApp::DxApp(const char*, int, int)':
  D:/_/B/src/OpenColorIO-2.6.0/src/libutils/oglapphelpers/dxapp.cpp:94:35: warning: missing initializer for member 'tagWNDCLASSEXA::style' [-Wmissing-field-initializers]
     94 |     WNDCLASSEXA windowClass = { 0 };
        |                                   ^

Note that in CMake if(WIN32) tests cover Windows as a platform in general (incl. MinGW), not a specific toolchain.