disks plugin: check for build prerequisites properly

plugin/disks/CMakeLists.txt

@@ -1,5 +1,7 @@
-IF("${CMAKE_SYSTEM}" MATCHES "Linux")
+INCLUDE (CheckIncludeFiles)
+CHECK_INCLUDE_FILES ("sys/statvfs.h;mntent.h" INFO_HEADERS LANGUAGE CXX)
+
+IF (INFO_HEADERS)
   INCLUDE_DIRECTORIES(${CMAKE_SOURCE_DIR}/sql)
   MYSQL_ADD_PLUGIN(DISKS information_schema_disks.cc MODULE_ONLY RECOMPILE_FOR_EMBEDDED)
 ENDIF()
-