Build: ggml-vulkan.cpp fails with "spirv/unified1/spirv.hpp: No such file" although find_package(SPIRV-Headers) succeeded #12

Repro (CachyOS Linux, gcc 16.2, CMake 4.4.4, Vulkan 1.4 headers + glslc installed, no system spirv-headers package; SPIRV-Headers installed to a local prefix passed via CMAKE_PREFIX_PATH):

Configure succeeds — find_package(SPIRV-Headers) resolves from the prefix and the Vulkan backend is enabled — but the build then fails:

ggml/src/ggml-vulkan/ggml-vulkan.cpp:48:14: fatal error: spirv/unified1/spirv.hpp: No such file or directory

The found package include directory is not propagated onto the ggml-vulkan target compile line (no -I<prefix>/include present).

Workaround: export CPATH=<prefix>/include before cmake --build.

This reproduces identically in skin-tokens.cpp (its patched-ggml FetchContent build hits the same error at the same line). On distros that ship spirv-headers system-wide it is invisible, which is probably why CI never sees it.

Suggested fix: link the imported SPIRV-Headers::SPIRV-Headers target (or forward its INTERFACE_INCLUDE_DIRECTORIES) to ggml-vulkan where the superbuild wires up the submodule — or document spirv-headers as a required system dependency next to the Vulkan loader in the README.

Verified 2026-10-18 on both repos, ggml submodule 8c63e70.