Be more careful when using SSE intrinsics.

Be more careful when using SSE intrinsics.

Summary:
- Check FOLLY_HAVE_EMMINTRIN_H before #including it in Range.cpp.
- The version of Clang that Mac OS X uses for Xcode is based on GCC 4.2.
It turns out that this does not have a full compliment of SSE2
builtins (e.g. __builtin_ia32_loaddqu is undefined, but
__builtin_ia32_storedqu is). Work around this omission by not
compiling the specialized code on this platform.

@override-unit-failures
Ignore unit test failures as two tests are just sitting in the postponed
state forever. I investigated all the other unit test failures and they
just look like flakey tests.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: delong.j@fb.com

FB internal diff: D999130