Use gcc4.7.1 on CentOS 5.2

Summary:
For some reason, snappy on CentOS 5.2 when compiled with gcc 4.8.1 segfaults on strcmp. (!?)

Add an if to compile with gcc4.7.1 if you're compiling on CentOS 5.2. Please update your devservers to CentOS 6.

Test Plan:
make clean; make check
on both my devserver (CentOS 6) and dhruba's (CentOS 5.2)

Reviewers: dhruba, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14169
Fix BZip constants

Summary: We were using ZLIB constants in BZIP code path. This caused some errors, like: https://github.com/facebook/rocksdb/issues/8

Test Plan: make clean; make check

Reviewers: dhruba, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14175
Fix stack overflow

Summary:
Sure, let me put 8 bytes in that int32_t.

Brought to you by ASAN!

Test Plan: ttl_test

Reviewers: dhruba, haobo, kailiu, emayanke

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14193
Remove libevent

Summary: We don't need that dependency

Test Plan: make check

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14199