CMake "Day 1" patch

[alexreinking]

Fixes several issues found on initial deployment of new CMake build, and adds a few new features:

• Clang_DIR-inferred-from-LLVM_DIR is now preferred when only LLVM_DIR is defined
• Binary size issues resolved
• Apps now print "Success!" messages to ensure they ran correctly and didn't just print 0-status usage messages (affected camera_pipeline)
  • Linear algebra tests now know their underlying CBlas implementation (missing command-line define)
• Option added for enabling WASM backend (no WASM tests, yet, though)
• Add multi-config packaging logic to Win32.
• Fixing Python apps that crash with host-cuda.

---

Regarding packaging, the intended workflow is to use a multi-config generator like Visual Studio or Ninja Multi-Config (currently broken on Windows, see this discussion on CMake Discourse).

Then you just:

> cmake --build . --config Debug
> cmake --build . --config Release
> cpack -C "Debug;Release"

[alexreinking]

Ninja Multi-Config bug was confirmed by maintainers. See https://gitlab.kitware.com/cmake/cmake/-/issues/20775

[alexreinking]

Looks like _alloca doesn't exist when you disable exceptions on MSVC. Odd. Including the <malloc.h> header seems to fix this. I pushed that fix and stopped the buildbots on the previous push.

[alexreinking]

Python build requires exceptions to be enabled in Halide. Otherwise the correctness test "test_compiletime_error" calls abort() which can't be caught by Python. Similarly with tuple_data and pystub.

@steven-johnson -- we should add HALIDE_ENABLE_EXCEPTIONS=YES to our buildbot flags. Alternatively, we can make it the default.