MDEV-8883 more cross-compiling fixes

don't abort with "XtraDB not supported" if XtraDB was disabled
in this build.

storage/xtradb/CMakeLists.txt

@@ -455,12 +455,12 @@ IF(WITH_INNODB)
   SET(WITH_INNOBASE_STORAGE_ENGINE TRUE)
 ENDIF()
 
-IF(XTRADB_OK)
-  MYSQL_ADD_PLUGIN(xtradb ${INNOBASE_SOURCES} STORAGE_ENGINE
+MYSQL_ADD_PLUGIN(xtradb ${INNOBASE_SOURCES} STORAGE_ENGINE
   DEFAULT
   RECOMPILE_FOR_EMBEDDED
   LINK_LIBRARIES ${ZLIB_LIBRARY} ${LINKER_SCRIPT})
-ELSE()
+
+IF(TARGET xtradb AND NOT XTRADB_OK)
   MESSAGE(FATAL_ERROR "Percona XtraDB is not supported on this platform")
 ENDIF()