Headers: exclude `#include_next <stdatomic.h>` on MSVC
The 14.31.30818 toolset has the following in the `stdatomic.h`:
#error <stdatomic.h> is not yet supported when compiling as C, but this is planned for a future release.
This results in clang failing to build existing code which relied on
`stdatomic.h` in C mode on Windows. Simply fallback to the clang header
until that header is available as a complete implementation.
1 file changed