Makefile had wrong ll stubs for gpu backends
Made lerp work in C codegen

by splitting off lerp lowering into an Expr into its own file
Renaming a derived dimension was broken.

Fixed it by rewriting old name in splits list in the derived dimension case.
Re-enabled test for vector multiply-add for float32 on arm.
Fix comment on how to run tests.
Remove dependency on system header file to allow cross compilation.
Further refine comment on how to run tests.
Merge branch 'master' of github.com:halide/Halide
Moved trivial for loop removal to fix vst2 generation test.
Updated INSTALL instructions
Made select do more intelligent casting of constants.
Fix bad comments on operator+=
Fail an assertion if halide_malloc returns non-null.
Temporarily remove non-null assertions due to failing test.
Add support for iOS code generation.
Fix typo in comment.
Fix to satisfy -Wpedantic
Avoid 64-bit division on 32-bit targets.
Halide routines now return an exit status.

0 == success. -1 == an assertion failed (and halide_error was called).

This allows for correct allocation cleanup on assertion fails inside nested
halide routines (e.g. parallel for loops, or the upcoming extern stages).
Can retrieve the internal parameter from an ImageParam.