Fix broken clang-cl compilation

- fix several Windows-specific "variable set but not used",
or "variable unused" warnings.
- correctly initialize std::atomic_flag (ATOMIC_FLAG_INIT)
- fix Ninja build for spider on Windows
- adjust check for sizeof(MYSQL) for Windows compilers

storage/innobase/include/fil0fil.h

@@ -422,7 +422,7 @@ private:
   bool being_imported= false;
 
   /** Whether any corrupton of this tablespace has been reported */
-  mutable std::atomic_flag is_corrupted{false};
+  mutable std::atomic_flag is_corrupted= ATOMIC_FLAG_INIT;
 
 public:
   /** mutex to protect freed_ranges and last_freed_lsn */
@@ -1527,7 +1527,10 @@ extern fil_system_t	fil_system;
 
 inline void fil_space_t::reacquire() noexcept
 {
-  ut_d(uint32_t n=) n_pending.fetch_add(1, std::memory_order_relaxed);
+#ifdef SAFE_MUTEX
+  uint32_t n=
+#endif
+  n_pending.fetch_add(1, std::memory_order_relaxed);
 #ifdef SAFE_MUTEX
   if (mysql_mutex_is_owner(&fil_system.mutex)) return;
   ut_ad(n & PENDING);

storage/spider/CMakeLists.txt

@@ -28,16 +28,5 @@ ELSE()
 
   MYSQL_ADD_PLUGIN(spider ${SPIDER_SOURCES} ${extra_options}
     STORAGE_ENGINE MODULE_ONLY)
-  IF(NOT TARGET spider)
-    RETURN()
-  ENDIF()
 ENDIF()
 
-IF(MSVC AND (TARGET spider))
-  IF (CMAKE_BUILD_TYPE STREQUAL "Debug")
-    ADD_CUSTOM_COMMAND(TARGET spider
-                       POST_BUILD
-                       COMMAND if not exist ..\\..\\sql\\lib mkdir ..\\..\\sql\\lib\\plugin
-                       COMMAND copy Debug\\ha_spider.dll ..\\..\\sql\\lib\\plugin\\ha_spider.dll)
-  ENDIF()
-ENDIF()