Add Halide::Type mapping to GeneratorParam

Add Halide::Type as a built-in option for GeneratorParam, for use in
customizing the width of input/output ImageParams. Also add a
customization to natural_vector_width() that takes a Halide::Type()
rather than a template parameter.
Augment extern_error test to set and test a specific value for user_context
Only build generators if CXX11 = true
Make generator tests respect HL_TARGET
Add missing EXPORT qualifier
Merge branch 'master' of https://github.com/Halide/halide
Fix race condition for C++ < C++11

Was crashing simd_op_check occasionally.

I'm a little suprised that Halide isn't thread-unsafe elsewhere
(e.g. unique_name, shared pointer reference counts) but I guess
if you never pass an Expr, Func, etc across threads, it all works.
Windows build fix
Merge branch 'master' into glsl-varyingattrib
GLSL does not support hexfloat format float literals.