Summary:
The fbcode compilation was always switching on msse by default.
This patch keeps the same behaviour but allows the compilation
process to switch off msse if needed.

If one does not want to use sse, then do the following:
export USE_SSE=0
make clean all

Test Plan: make clean all

Reviewers: heyongqiang

Reviewed By: heyongqiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D6717