how to use #ifdef with an OR condition?

Like this

#if defined(LINUX) || defined(ANDROID)

OR condition in #ifdef

#if defined LINUX || defined ANDROID
// your code here
#endif /* LINUX || ANDROID */

or-

#if defined(LINUX) || defined(ANDROID)
// your code here
#endif /* LINUX || ANDROID */

Both above are the same, which one you use simply depends on your taste.

P.S.: #ifdef is simply the short form of #if defined, however, does not support complex condition.

Further-