Repository to build wheels

We are using this repo to build wheels for symengine.py. Instead of using the multibuild infrastructure, we are using conda-forge infrastructure because of few reasons

1. Dependencies like LLVM which take a lot of time are pre-built.
2. cross-compiling is supported.

In order to use conda-forge built dependencies, there are a few catches due to the use of recent libgcc and libsdtcxx. In order to do that, we need to do the following for Linux,

1. If C shared libraries are used, they need to be built with -static-libgcc or cross our fingers that they don't need a newer version of libgcc. (We checked and they didn't) Else use static libraries.
2. Use only static libraries for C++ dependencies.
3. Build the wrapper with -static-libgcc -static-libstdcxx

For macOS we need to do the following,

1. Use only static libraries for C++ dependencies.
2. Link in libc++ statically or hope that the dependencies don't use newer features of libc++. (We checked and they didn't)

For Windows,

1. Hope that the user has install Visual C++ 2017 Redistributable.