ggml : Parallelize quant LUT init (#23595)

- Use OpenMP to parallelize iq2xs_init_impl and iq3xs_init_impl.
- Move the OpenMP detection from ggml-cpu to ggml-base.
- Update OpenMP dependencies in ggml-config.cmake.in.

ggml/src/CMakeLists.txt

@@ -222,6 +222,23 @@ if (GGML_SCHED_NO_REALLOC)
     target_compile_definitions(ggml-base PUBLIC GGML_SCHED_NO_REALLOC)
 endif()
 
+if (GGML_OPENMP)
+    find_package(OpenMP)
+    if (OpenMP_FOUND)
+        set(GGML_OPENMP_ENABLED "ON" CACHE INTERNAL "")
+    else()
+        set(GGML_OPENMP_ENABLED "OFF" CACHE INTERNAL "")
+        message(WARNING "OpenMP not found")
+    endif()
+else()
+    set(GGML_OPENMP_ENABLED "OFF" CACHE INTERNAL "")
+endif()
+
+if (GGML_OPENMP_ENABLED)
+    target_compile_definitions(ggml-base PRIVATE GGML_USE_OPENMP)
+    target_link_libraries(ggml-base PRIVATE OpenMP::OpenMP_C OpenMP::OpenMP_CXX)
+endif()
+
 add_library(ggml
             ggml-backend-dl.cpp
             ggml-backend-reg.cpp)