MDEV-5706 MariaDB does not build on hurd-i386

backport from 10.0 the fix for:
  MDEV-5756 CMake option to build without thread pool

sql/CMakeLists.txt

@@ -30,7 +30,15 @@ ${CMAKE_CURRENT_BINARY_DIR}/lex_hash.h
 
 SET_SOURCE_FILES_PROPERTIES(${GEN_SOURCES} PROPERTIES GENERATED 1)
 
-ADD_DEFINITIONS(-DMYSQL_SERVER -DHAVE_EVENT_SCHEDULER -DHAVE_POOL_OF_THREADS) 
+ADD_DEFINITIONS(-DMYSQL_SERVER -DHAVE_EVENT_SCHEDULER) 
+
+IF (CMAKE_SYSTEM_NAME MATCHES "Linux" OR
+    CMAKE_SYSTEM_NAME MATCHES "Windows" OR
+    CMAKE_SYSTEM_NAME MATCHES "SunOS" OR
+    HAVE_KQUEUE)
+ ADD_DEFINITIONS(-DHAVE_POOL_OF_THREADS) 
+ENDIF()
+
 IF(SSL_DEFINES)
  ADD_DEFINITIONS(${SSL_DEFINES})
 ENDIF()

sql/scheduler.h

@@ -99,14 +99,13 @@ public:
   void *data;                  /* scheduler-specific data structure */
 };
 
-#if !defined(EMBEDDED_LIBRARY)
-#define HAVE_POOL_OF_THREADS 1
+#ifdef HAVE_POOL_OF_THREADS
 void pool_of_threads_scheduler(scheduler_functions* func,
    ulong *arg_max_connections,
    uint *arg_connection_count);
 #else
 #define pool_of_threads_scheduler(A,B,C) \
   one_thread_per_connection_scheduler(A, B, C)
-#endif
+#endif /*HAVE_POOL_OF_THREADS*/
 
 #endif /* SCHEDULER_INCLUDED */