fix build bug from recent commit:https://github.com/facebook/rocksdb/commit/43c386b72ee834c88a1a22500ce1fc36a8208277
tweak build bug fix
docs for shared library builds
Merge pull request #48 from dyu/master

fix build bug
Add -DROCKSDB_FALLOCATE_PRESENT to fbcode build
moving autovector_test after db_test
Support multi-threaded DisableFileDeletions() and EnableFileDeletions()

Summary:
We don't want two threads to clash if they concurrently call DisableFileDeletions() and EnableFileDeletions(). I'm adding a counter that will enable file deletions only after all DisableFileDeletions() calls have been negated with EnableFileDeletions().

However, we also don't want to break the old behavior, so I added a parameter force to EnableFileDeletions(). If force is true, we will still enable file deletions after every call to EnableFileDeletions(), which is what is happening now.

Test Plan: make check

Reviewers: dhruba, haobo, sanketh

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14781