Python bindings use RTTI, but libHalide is built with -fno-rtti

[steven-johnson]

The python bindings use Boost features which require RTTI be enabled, but the default build environment links with libHalide which is (and has always been) built with RTTI disabled.

Amazingly, this has happened to work fairly reliably up till now, but is intrinsically fragile (since the compiler is free to assume that typeinfo is defined in libHalide, which it isn't).

(It's still working in trunk at this time, but I have a private branch that seems to have tickled this into breaking, due to addition of virtual methods to a class which previously didn't have them.)

Not sure if there is a good solution here:

• Boost absolutely requires RTTI
• Not clear if there are acceptable alternatives to Boost for C++/Python bindings (e.g., SWIG is terrible)
• Halide needs to support non-RTTI builds, and would prefer not to add RTTI builds to the mix

[alexreinking]

CMake currently requires that RTTI is enabled for both LLVM and Halide when building the Python bindings. These checks were introduced in #4644

Moreover we are no longer using Boost, but pybind11, for generating the bindings. Thus I am marking this issue as resolved.