CMake: Use CMAKE_SYSTEM_NAME STREQUAL "WASI" instead of WASI

The `WASI` variable is only available in the bleeding-edge CMake and not
released versions yet
(https://gitlab.kitware.com/cmake/cmake/-/merge_requests/9659).
So, we should use the old way for now.

Author: kateinoigakukun

cmake/modules/shared/CompilerSettings.cmake

@@ -27,7 +27,7 @@ endif()
 if(NOT APPLE)
   add_compile_definitions("SWT_NO_SNAPSHOT_TYPES")
 endif()
-if(WASI)
+if(CMAKE_SYSTEM_NAME STREQUAL "WASI")
   add_compile_definitions("SWT_NO_DYNAMIC_LINKING")
   add_compile_definitions("SWT_NO_PIPES")
 endif()