s/WITH_PNACL/WITH_NATIVE_CLIENT/ in a few places

This prevented specifying GPU targets with PNaCl targets
Merge pull request #553 from halide/WITH_NATIVE_CLIENT

s/WITH_PNACL/WITH_NATIVE_CLIENT/ in a few places
EXPORTed operator<<(ostream, Stmt)
Added EXPORT to another defininition of operator<<(ostream, Stmt)
Added get_target_triple to CodeGen_PNaCl.
PNACL -> PNaCl
Move NamesInterface from Internal to Halide namespace
Merge branch 'master' of https://github.com/halide/Halide
Add Generator::get_filter_parameters()
Merge branch 'master' of https://github.com/halide/Halide
Change int -> size_t in GeneratorBase::get_filter_parameters()
Update test_all_targets.sh to use libpng1.6.14
Added correctness test for GLSL varying attributes.
Avoid unletifying inside linear expressions.
Parameter::get_scalar() should be const
Added error checks and workaround comments to UnvectorizeIntrinsicArg
Added tests for varying attributes and for GL look up table patterns.
Merge branch 'master' into glsl-varyingattrib