Summary:
clang is an alternate compiler based on llvm.  It produces
nicer error messages and finds some bugs that gcc doesn't, such as the
size_t change in this file (which caused some write return values to be
misinterpreted!)

Clang isn't the default; to try it, do "USE_CLANG=1 make" or "export
USE_CLANG=1" then make as normal

Test Plan: "make check" and "USE_CLANG=1 make check"

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D7899