use optional compile flags only when runtime cpu is on (#5827)

cmake/ncnn_add_layer.cmake

@@ -49,7 +49,9 @@ macro(ncnn_add_arch_opt_source class NCNN_TARGET_ARCH_OPT NCNN_TARGET_ARCH_OPT_C
     set(NCNN_${NCNN_TARGET_ARCH_OPT}_SOURCE ${CMAKE_CURRENT_SOURCE_DIR}/layer/${NCNN_TARGET_ARCH}/${name}_${NCNN_TARGET_ARCH}_${NCNN_TARGET_ARCH_OPT}.cpp)
 
     if(WITH_LAYER_${name} AND EXISTS ${NCNN_${NCNN_TARGET_ARCH_OPT}_SOURCE})
-        set_source_files_properties(${NCNN_${NCNN_TARGET_ARCH_OPT}_SOURCE} PROPERTIES COMPILE_FLAGS ${NCNN_TARGET_ARCH_OPT_CFLAGS})
+        if(NCNN_RUNTIME_CPU)
+            set_source_files_properties(${NCNN_${NCNN_TARGET_ARCH_OPT}_SOURCE} PROPERTIES COMPILE_FLAGS ${NCNN_TARGET_ARCH_OPT_CFLAGS})
+        endif()
         list(APPEND ncnn_SRCS ${NCNN_${NCNN_TARGET_ARCH_OPT}_SOURCE})
     endif()
 endmacro()
@@ -70,8 +72,9 @@ macro(ncnn_add_arch_opt_layer_source class NCNN_TARGET_ARCH_OPT_BASE NCNN_TARGET
         )
         set_source_files_properties(${NCNN_${NCNN_TARGET_ARCH_OPT_BASE}_${NCNN_TARGET_ARCH_OPT}_SOURCE} PROPERTIES GENERATED TRUE)
 
-        set_source_files_properties(${NCNN_${NCNN_TARGET_ARCH_OPT_BASE}_${NCNN_TARGET_ARCH_OPT}_SOURCE} PROPERTIES COMPILE_FLAGS ${NCNN_TARGET_ARCH_OPT_CFLAGS})
-
+        if(NCNN_RUNTIME_CPU)
+            set_source_files_properties(${NCNN_${NCNN_TARGET_ARCH_OPT_BASE}_${NCNN_TARGET_ARCH_OPT}_SOURCE} PROPERTIES COMPILE_FLAGS ${NCNN_TARGET_ARCH_OPT_CFLAGS})
+        endif()
         list(APPEND ncnn_SRCS ${NCNN_${NCNN_TARGET_ARCH_OPT_BASE}_${NCNN_TARGET_ARCH_OPT}_SOURCE})
     endif()
 endmacro()