Add faster, lower precision variants of the standard math routines. These are enabled by default

The system math library can be reverted to with USE_FAST_MATH=0
Make it build on llvm 3.5
Fix signed/unsigned comparisons
testsuite ref output changes after recent transcendantal approximations
More testsuite reference output updates
Another round of careful updates to testsuite reference output.
At this point, both OSX and Linux pass 100% of tests for me!
CHANGES
bump dev release version
RendererServices API overhaul -- all the texture functions have been
slightly changed to explicitly pass nchannels and the derivative pointers
rather than embedding those in the TextureOpt struct. This goes along with
an OIIO-side change that altered the TextureSystem API in a similar way and
eliminated those fields from TextureOpt. This was part of a big overhaul
aimed at SSE-izing the TextureSystem internals.
OSL-side changes corresponding to SIMD overhaul of TextureSystem.

TextureSystem has eliminated the nchannels and derivs field from
TextureOpt, instead passing those explicitly to the texture API functions.
We change here to match. Also, we make use of the new OIIO::simd::float4
class, to exploit SSE and other instruction-level SIMD in the glue code
surrounding the texture calls.
simd Make/cmake
Bump SPI minimal SSE level to 4.1 on spinux, 4.2 on OSX