cmake : avoid duplicate -march=native when already set in CFLAGS

If -march= is already specified via CMAKE_C_FLAGS/CMAKE_CXX_FLAGS or
ARCH_FLAGS, do not append -march=native again. This prevents compiler
warnings about redundant flags and allows users to override the march
target by setting it in their build environment.

Fixes duplicate -march flag in ggml-cpu and ggml-amx backends.

Author: stone

ggml/src/ggml-amx/CMakeLists.txt

@@ -61,7 +61,16 @@ if (CMAKE_OSX_ARCHITECTURES STREQUAL "x86_64" OR CMAKE_GENERATOR_PLATFORM_LWR MA
         endif()
     else()
         if (GGML_NATIVE)
-            list(APPEND ARCH_FLAGS -march=native)
+            set(march_flag_found FALSE)
+            foreach(flag IN LISTS ARCH_FLAGS)
+                if(flag MATCHES "^-march=")
+                    set(march_flag_found TRUE)
+                     break()
+                endif()
+            endforeach()
+            if(NOT march_flag_found)
+               list(APPEND ARCH_FLAGS -march=native)
+            endif()
         endif()
         if (GGML_F16C)
             list(APPEND ARCH_FLAGS -mf16c)

ggml/src/ggml-cpu/CMakeLists.txt

@@ -235,7 +235,10 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
             endif()
         else ()
             if (GGML_NATIVE)
-                list(APPEND ARCH_FLAGS -march=native)
+               string(REGEX MATCH "-march=[^ ]+" MARCH_MATCH "${CMAKE_C_FLAGS} ${CMAKE_CXX_FLAGS}")
+               if(NOT MARCH_MATCH)
+                  list(APPEND ARCH_FLAGS -march=native)
+               endif()
             else ()
                 list(APPEND ARCH_FLAGS -msse4.2)
                 list(APPEND ARCH_DEFINITIONS GGML_SSE42)