Don't invoke CPUID with feature levels above the supported maximum.
In this case CPUID will return the highest supported basic information
leaf, which will have unrelated bits in the relevant positions.
Apparently, the presence of `cpuid.h` is not necessarily sufficient to
guarantee the availability of `__cpuid_count()`. We therefore test for
the latter explicitly.
By usage of -fmodules, all the intrinsic items are loaded automatically.
The headers included come however from Visual Studio, thus bringing some
conflicting declarations. On the other hand, -fmodules is needed to
mitigate linking issues with clang in VS compatibility mode.
