Add safety check to Target::to_string
Allow selection of ACCELERATOR devices.
Merge pull request #452 from richardmembarth/acc

Allow selection of ACCELERATOR devices.
Remove comma at end of enum

-pedantic requires it
Handle a*b - c*d using pmaddwd when appropriate
Merge branch 'master' of github.com:halide/Halide
Fix usage of Target constructor on arm, mips
Change LLVM version test in Makefile

Fixes nightly build for llvm 3.2 on linux. Original didn't work for me for
reasons I can't figure out. Went with brute-force alternative.
This change finishes plumbing through MIPS code generation support.

The HelloAndroid application is extended to build for multiple
architectures, including ARMv6, ARMv7a, 64-bit ARM, 32-bit MIPS,
32-bit x86 and 64-bit x86. (64-bit MIPS is not yet supported.)

GPU are added templates for all architectures. E.g. so GLSL can be
used from PNaCl and on mips

The usage of WITH_ preprocessor macros has been changed to consistently use #ifdef, not #if.

Added a README for building the HelloHalide app.
Merge branch 'master' into compute_cached
Fix Makefile. Add comment to explain how WITH_ flags work.