only use brace initialization for clGetPlatformIDs when anonymous unions are supported

[bashbaug]

We hit a problem recently in the OpenCL-CLHPP CI where OpenCL ICD loader builds with CMAKE_C_EXTENSIONS=OFF produced anonymous union / brace initialization warnings. Because the CI builds with warnings-as-errors, this caused CI to fail.

KhronosGroup/OpenCL-CLHPP#330

/home/bashbaug/git/OpenCL-ICD-Loader/loader/icd_dispatch_generated.c:6828:5: warning: braces around scalar initializer
 6828 |     {&clGetPlatformIDs_disp},

At least on my system, the difference between CMAKE_C_EXTENSIONS=OFF and the default CMAKE_C_EXTENSIONS=ON is that compilation with extensions off passes -std=c99, whereas compilation with extensions on is done with -std=gnu99.

If we want to build with extensions off and hence with -std=c99 we should only use brace initialization when anonymous unions are supported. This may be as simple as something like:

struct _cl_icd_dispatch khrMasterDispatch = {
+#if __CL_HAS_ANON_STRUCT__
    {
+#endif
    &clGetPlatformIDs_disp
+#if __CL_HAS_ANON_STRUCT__
    }
+#endif
    ,
...