MDEV-14469 build with cmake -DMYSQL_MAINTAINER_MODE=ON fails: 'readdir_r' is deprecated

1. test readdir_r() availability under -Werror
2. don't protect readdir() with mutexes, it's not needed for the way
   we use readdir()

configure.cmake

@@ -407,7 +407,6 @@ CHECK_FUNCTION_EXISTS (pthread_sigmask HAVE_PTHREAD_SIGMASK)
 CHECK_FUNCTION_EXISTS (pthread_threadmask HAVE_PTHREAD_THREADMASK)
 CHECK_FUNCTION_EXISTS (pthread_yield_np HAVE_PTHREAD_YIELD_NP)
 CHECK_FUNCTION_EXISTS (putenv HAVE_PUTENV)
-CHECK_FUNCTION_EXISTS (readdir_r HAVE_READDIR_R)
 CHECK_FUNCTION_EXISTS (readlink HAVE_READLINK)
 CHECK_FUNCTION_EXISTS (re_comp HAVE_RE_COMP)
 CHECK_FUNCTION_EXISTS (regcomp HAVE_REGCOMP)
@@ -465,6 +464,16 @@ IF(HAVE_SYS_EVENT_H)
 CHECK_FUNCTION_EXISTS (kqueue HAVE_KQUEUE)
 ENDIF()
 
+# readdir_r might exist, but be marked deprecated
+SET(CMAKE_REQUIRED_FLAGS -Werror)
+CHECK_CXX_SOURCE_COMPILES(
+"#include  <dirent.h>
+int main() {
+  readdir_r(0,0,0);
+  return 0;
+  }" HAVE_READDIR_R)
+SET(CMAKE_REQUIRED_FLAGS)
+
 #--------------------------------------------------------------------
 # Support for WL#2373 (Use cycle counter for timing)
 #--------------------------------------------------------------------

mysys/my_lib.c

@@ -103,10 +103,6 @@ MY_DIR	*my_dir(const char *path, myf MyFlags)
   DBUG_ENTER("my_dir");
   DBUG_PRINT("my",("path: '%s' MyFlags: %d",path,MyFlags));
 
-#if !defined(HAVE_READDIR_R)
-  mysql_mutex_lock(&THR_LOCK_open);
-#endif
-
   dirp = opendir(directory_file_name(tmp_path,(char *) path));
 #if defined(__amiga__)
   if ((dirp->dd_fd) < 0)			/* Directory doesn't exists */
@@ -162,9 +158,6 @@ MY_DIR	*my_dir(const char *path, myf MyFlags)
   }
 
   (void) closedir(dirp);
-#if !defined(HAVE_READDIR_R)
-  mysql_mutex_unlock(&THR_LOCK_open);
-#endif
   result->dir_entry= (FILEINFO *)dir_entries_storage->buffer;
   result->number_off_files= dir_entries_storage->elements;
   
@@ -174,9 +167,6 @@ MY_DIR	*my_dir(const char *path, myf MyFlags)
   DBUG_RETURN(result);
 
  error:
-#if !defined(HAVE_READDIR_R)
-  mysql_mutex_unlock(&THR_LOCK_open);
-#endif
   my_errno=errno;
   if (dirp)
     (void) closedir(dirp);