There is a tiny inconvenience in OpenCL.std.h included by libspirv.h. That OpenCL header file is a C++ header file and so a client including libspirv.h cannot compile as C application, but must use a C++ compiler. (msvc complains when compiling a .c file including spirv-tools\libspirv.h)
Simply guarding the "namespace" directive with an #ifdef __cplusplus , or removing that namespace declaration, helped.