[FEA] Add initial CMakeLists.txt

[trxcllnt]

edit: Closing in favor of #514

This PR adds a CMakeLists.txt file for configuring and building with CMake:

cmake -S . -B build -GNinja \
  -D CMAKE_CUDA_ARCHITECTURES=NATIVE \
  -D legate_core_ROOT=/path/to/legate/core/build

This PR depends on the legate.core CMake PR, as well as this branch in my Legion fork:

# Build legate.core with a custom fork of Legion
cmake -S . -B build -GNinja \
  -D CMAKE_CUDA_ARCHITECTURES=NATIVE \
  -D Legion_USE_CUDA=ON \
  -D Legion_USE_OpenMP=ON \
  -D Legion_USE_GASNet=ON \
  -D GASNet_CONDUIT=mpi \
  -DLEGATE_CORE_LEGION_BRANCH=fix/use-as-cmake-submodule-master \
  -DLEGATE_CORE_LEGION_REPOSITORY=https://gitlab.com/inlineptx/legion.git

Or to build cuNumeric, legate.core, and legion all at once:

git clone https://github.com/trxcllnt/cunumeric.git --branch=fea/add-cmake

cd cunumeric

cmake -S . -B build -GNinja \
  -D GASNet_CONDUIT=mpi \
  -D Legion_USE_CUDA=ON \
  -D Legion_USE_OpenMP=ON \
  -D Legion_USE_GASNet=ON \
  -D CMAKE_CUDA_ARCHITECTURES=NATIVE \
  -D LEGATE_CORE_LEGION_BRANCH=fix/use-as-cmake-submodule \
  -D LEGATE_CORE_LEGION_REPOSITORY=https://github.com/trxcllnt/legion.git \
  -D CUNUMERIC_LEGATE_CORE_BRANCH=fea/add-cmake \
  -D CUNUMERIC_LEGATE_CORE_REPOSITORY=https://github.com/trxcllnt/legate.core

cmake --build build

Depends on nv-legate/legate#198 and https://gitlab.com/StanfordLegion/legion/-/merge_requests/502 edit: 502 has been merged