Add support for including oneCCL in a CMake mono build

[sogartar]

It would be nice to be able to include oneCCL inside another build directly.

add_subdirectory(oneCCL)

This entails that all CMake configuration arguments should be prefixed by something like ONECCL_ to avoid name conflicts.
Also all targets should be prefixed. E.g. oneccl::.

Another thing is to allow for all dependencies like googletest to be excludable from the build an provided externally. They may be defined somewhere else in the super build. E.g. ONECCL_GOOGLE_TEST_EXTERNAL=ON.

[nikitaxgusev]

Hello @sogartar, we'd like to get your case, is it possible for you to show how and which project you include oneccl to reproduce this issue?

[sogartar]

This is more of a feature request.
Nowadays it is very common for CMake projects to have a superbuild where most/all its dependencies are included in the superbuild and built together with the project.
To achieve this properly you also need to be able to exclude your dependency from your superbuild but have the provided from outside to prevent conflicts with other dependencies' superbuild.
A classic example is gtest because it is often included in the superbuild. OneCCL is also doing this here https://github.com/oneapi-src/oneCCL/blob/9e66a9e39ad643aad6f4997a98e2b69a6338c2bb/tests/functional/CMakeLists.txt#L65
It technically has a superbuild.
It should be something like

if(NOT ONECCL_GOOGLE_TEST_EXTERNAL)
  set(GTEST_DIR ${PROJECT_SOURCE_DIR}/../googletest-release-1.8.1/googletest)
  add_subdirectory(${GTEST_DIR} gtest_build)
if()

I also see there are things like

set(INC_DIRS
    ${GTEST_DIR}/include
    ${GTEST_DIR}/src
    ${EXAMPLES_DIR}/include)

include_directories(${INC_DIRS})

which is not the proper way to do it in CMake >= 3.0. You should use targets wherever possible and let the target provide its include directories and link libraries.

target_link_libraries(mytarget PRIVATE gtest)