various platforms; improve android port speed.

Avoid static initializer by using a new portability interface for
thread-safe lazy initialization.  Custom ports will need to be
extended to implement InitOnce/OnceType/LEVELDB_ONCE_INIT.

Fix endian-ness detection (fixes Powerpc builds).

Build related fixes:
- Support platforms that have unversioned shared libraries.
- Fix IOS build rules.

Android improvements
- Speed up atomic pointers
- Share more code with port_posix.

Do not spin in a tight loop attempting compactions if the file system
is inaccessible (e.g., if kerberos tickets have expired or if it is out
of space).

In particular, we add a new FilterPolicy class.  An instance
of this class can be supplied in Options when opening a
database.  If supplied, the instance is used to generate
summaries of keys (e.g., a bloom filter) which are placed in
sstables.  These summaries are consulted by DB::Get() so we
can avoid reading sstable blocks that are guaranteed to not
contain the key we are looking for.

This change provides one implementation of FilterPolicy
based on bloom filters.

Other changes:
- Updated version number to 1.4.
- Some build tweaks.
- C binding for CompactRange.
- A few more benchmarks: deleteseq, deleterandom, readmissing, seekrandom.
- Minor .gitignore update.

- Added a C binding for LevelDB.
  May be useful as a stable ABI that can be used by 
  programs that keep leveldb in a shared library, 
  or for JNI API.

- Replaced SQLite's readseq benchmark to a more efficient version. 
  SQLite readseq speeds increased by about a factor of 2x 
  from the previous version. Also updated benchmark page to
  reflect readseq speed up.



git-svn-id: https://leveldb.googlecode.com/svn/trunk@46 62dab493-f737-651d-591e-8d6aee1b9529