Summary:
ROCKSDB_WARNING is only defined if either ROCKSDB_PLATFORM_POSIX or OS_WIN is defined.  This works well for building rocksdb with its own build scripts.  But this won't work when an outside project(like mongodb) doesn't define ROCKSDB_PLATFORM_POSIX.

This fix defines ROCKSDB_WARNING for all platforms.  No idea if its defined correctly on non-posix,non-windows platforms but this is no worse that the current situation where this macro is missing on unexpected platforms.

This fix should hopefully fix anyone whose build broke now that we've switched from using #warning to Pragma (to support windows).  Unfortunately, while mongo-rocks compiles, it ignores the Pragma and doesn't print a warning.  I have not been able to figure out a way to implement this portably on all platforms.

Of course, an alternate solution would be to just get rid of ROCKSDB_WARNING and live with include file redirects indefinitely.  Thoughts?

Test Plan: build rocks, build mongorocks

Reviewers: igor, kradhakrishnan, IslamAbdelRahman, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D42477