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/ggml-cpu/CMakeLists.txt

@@ -72,17 +72,9 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
         endif()
     endif()
 
-    if (GGML_OPENMP)
-        find_package(OpenMP)
-        if (OpenMP_FOUND)
-            set(GGML_OPENMP_ENABLED "ON" CACHE INTERNAL "")
-            target_compile_definitions(${GGML_CPU_NAME} PRIVATE GGML_USE_OPENMP)
-
-            target_link_libraries(${GGML_CPU_NAME} PRIVATE OpenMP::OpenMP_C OpenMP::OpenMP_CXX)
-        else()
-            set(GGML_OPENMP_ENABLED "OFF" CACHE INTERNAL "")
-            message(WARNING "OpenMP not found")
-        endif()
+    if (GGML_OPENMP_ENABLED)
+        target_compile_definitions(${GGML_CPU_NAME} PRIVATE GGML_USE_OPENMP)
+        target_link_libraries(${GGML_CPU_NAME} PRIVATE OpenMP::OpenMP_C OpenMP::OpenMP_CXX)
     endif()
 
     if (GGML_LLAMAFILE)