Failed to generate build files on clang GNU frontend on Windows with `-DJPEGXL_STATIC=ON`

**Describe the bug**

After #3837, CMake can no longer produce build files due to failing atomics check. This only happens with Clang with GNU driver and `-DJXL_STATIC=ON`. This means, MSVC and Clang with MSVC driver (clang-cl) are not affected, and Clang with GNU driver with `-DJXL_STATIC=OFF` is not affected.

```

-- The C compiler identification is Clang 19.1.0 with GNU-like command-line

-- The CXX compiler identification is Clang 19.1.0 with GNU-like command-line

...

-- Performing Test ATOMICS_IN_LIBRARY

-- Performing Test ATOMICS_IN_LIBRARY - Failed

CMake Error at cmake/FindAtomics.cmake:49 (message):

  Neither lock free instructions nor -latomic found.

Call Stack (most recent call first):

  C:/Users/Alifian/scoop/apps/vcpkg/current/scripts/buildsystems/vcpkg.cmake:859 (_find_package)

  CMakeLists.txt:262 (find_package)

```



**To Reproduce**

Steps to reproduce the behavior:

Use `-DJPEGXL_STATIC=ON` together with Clang-GNU driver (`-DCMAKE_CXX_COMPILER=clang++`)



**Environment**

 - OS: [e.g. Windows 11 23H2]

 - Compiler version: Clang 19.1.0

 - CPU type: x86_64

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