[FEA] Add initial CMakeLists.txt

[trxcllnt]

edit: Closing in favor of #323

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

$ cmake \
    -S . \
    -B build \
    -G Ninja \
    -D CMAKE_CUDA_ARCHITECTURES=NATIVE \
    -D Legion_MAX_DIM=3 \
    -D Legion_MAX_FIELDS=256 \
    -D Legion_USE_CUDA=ON \
    -D Legion_USE_LLVM=ON \
    -D Legion_USE_HDF5=OFF \
    -D Legion_USE_OpenMP=ON \
    -D Legion_USE_GASNet=OFF \
    -D Legion_USE_Python=OFF \
    -D Legion_CUDA_DYNAMIC_LOAD=ON \
    -D Legion_BOUNDS_CHECKS=ON \
    -D Legion_HIJACK_CUDART=OFF

$ cmake --build build

The CMakeLists.txt uses CPM and rapids-cmake to find dependencies already installed on the system, or download+build them from source. CPM will ensure dependencies (such as legion) are built as part of building legate.core.

With a properly configured dependency tree, CMake's find_package can also use a dependency's build directory as a package root:

# Build legion from the following local directory
cmake -S . -B build -D CPM_Legion_SOURCE=/path/to/legion

# Or use legion's build directory as the package root
cmake -S . -B build -D Legion_ROOT=/path/to/legion/build

Opening as a draft PR while I implement and test all the functionality presently in install.py.

Depends on https://gitlab.com/StanfordLegion/legion/-/merge_requests/502 edit: 502 has been merged

[manopapad]

Exciting! A couple of points to keep in mind, as you make changes to the build system:

Legion has a cmake workflow, that you possibly want to plug in. Note, however, that the Legion make and cmake workflows are not necessarily at parity. Legate has only been using the make workflow, so we may have added some features to that workflow that don't exit in the cmake one.

Please make sure that builds can be made incrementally, and dependencies will carry over all the way from Legion to legate.core to cunumeric. For example:

• legion-build-dir/foo.o should not be updated if legion-src-dir/foo.c and any referenced header files have not changed.
• legion-install-dir/lib/liblegion.so should not be updated if none of the object files comprising it have changed.
• legion-install-dir/include/foo.h should not be updated if legion-src-dir/foo.h has not changed.
• Header file dependencies should be recorded. I don't know how cmake does this, but in the Legion & legate Makefiles I used -MMD and similar compiler options.

Apologies if this all sounds obvious, but it wasn't trivial to get all of this working properly with make, and supporting multiple compilers with different supported -MMD-like options.

Note that Legion, Realm and legate.core record some of their configuration choices in a header file (legion_defines.h, realm_defines.h and config.mk respectively), e.g. whether this is a release or debug build. Everything downstream (recursively) depends on these header files, so it is critical that these are not updated if the build options have not changed.

[jjwilke]

not sure if it's worth documenting where this requirement comes from inline in the file with a comment