MDEV-5706 MariaDB does not build on hurd-i386

Followup. Don't compile threadpool_unix.cc when thread pool is disabled

sql/CMakeLists.txt

@@ -32,13 +32,6 @@ SET_SOURCE_FILES_PROPERTIES(${GEN_SOURCES} PROPERTIES GENERATED 1)
 
 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()
@@ -97,10 +90,16 @@ SET (SQL_SOURCE
                ${MYSYS_LIBWRAP_SOURCE}
 			   )
   
-IF(WIN32)
-  SET(SQL_SOURCE ${SQL_SOURCE} threadpool_win.cc)
-ELSE()
- SET(SQL_SOURCE ${SQL_SOURCE} threadpool_unix.cc)
+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) 
+ IF(WIN32)
+   SET(SQL_SOURCE ${SQL_SOURCE} threadpool_win.cc)
+ ELSE()
+   SET(SQL_SOURCE ${SQL_SOURCE} threadpool_unix.cc)
+ ENDIF()
 ENDIF()
 
 MYSQL_ADD_PLUGIN(partition ha_partition.cc STORAGE_ENGINE DEFAULT STATIC_ONLY