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