cmake: reuse existing asio target if present

[kkkzbh]

I’m using standalone Asio via vcpkg in an existing project, with CMake configured like this:

find_package(asio CONFIG REQUIRED)

When I tried to integrate stdexec and asioexec into the same project using CPM:

CPMAddPackage(
    NAME stdexec
    GITHUB_REPOSITORY NVIDIA/stdexec
    GIT_TAG main
    OPTIONS
        "STDEXEC_ENABLE_ASIO ON"
        "STDEXEC_ASIO_IMPLEMENTATION standalone"
)

CMake failed with the following error:

CMake Error at /home/kkkzbh/vcpkg/scripts/buildsystems/vcpkg.cmake:649 (_add_library):
  _add_library cannot create target "asio" because an alias with the same
  name already exists.
Call Stack (most recent call first):
  cmake-build-debug/_deps/stdexec-src/cmake/import_standalone_asio.cmake:29 (add_library)
  cmake-build-debug/_deps/stdexec-src/CMakeLists.txt:524 (import_standalone_asio)

This happens because vcpkg already provides an asio target, and stdexec’s CMake code tries to create another asio target in import_standalone_asio.cmake, which CMake rejects.

There are a few ways to work around this in user code:

• Switch to the Boost.Asio backend for stdexec, or
• Remove the find_package(asio CONFIG REQUIRED) + vcpkg-provided Asio and only use the asio target created by stdexec.

However, it is more convenient and less surprising if stdexec can reuse an existing asio target instead of failing.

To avoid the conflict, I changed the import_standalone_asio function to detect an existing asio target and reuse it:

function(import_standalone_asio)
    if (TARGET asio)
        message(STATUS "stdexec: reusing existing asio target")
        return()
    endif()

    set(options "")
    # ...
endfunction()

With this change:

• If a project (for example, via vcpkg) already defines an asio target, stdexec will reuse it and skip creating its own target.
• If no asio target exists, stdexec behaves as before and imports standalone Asio normally.

This makes it easier to integrate stdexec into existing CMake projects that already depend on standalone Asio.

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[ericniebler]

/ok to test 7f20bea

[ericniebler]

@kkkzbh are there more changes you plan to make to this PR? i notice it is still only a draft PR.