Simplify the process of using NVTX in another CMake project

[alliepiper]

Rather than requesting users use a custom include(...) file to consume imported CMake targets, it is possible to detect whether NVTX is being built stand-alone or as part of another project. The common pattern for detecting this is to add a top-level CMakeLists.txt file with:

In a new NVTX/CMakeLists.txt file:

# ...normal project setup here...

# Check if standalone or part of another project:
if ("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_LIST_DIR}")
  set(NVTX_TOPLEVEL_PROJECT ON)
endif()

# using `option` instead of `set` lets users change this using `ccmake` or `cmake -D....`.
# When built as part of another project, NVTX_TOPLEVEL_PROJECT will be unset and default to false.
# When built stand-alone, NVTX_TOPLEVEL_PROJECT will be truthy and disable imported targets.
option(NVTX3_TARGETS_NOT_USING_IMPORTED "<doc string>" ${NVTX_TOPLEVEL_PROJECT})

# The `set(...NOT_USING_IMPORTED...)` in c/CMakeLists.txt should be removed in favor of the option above.
add_subdirectory(c)

For an example of how we use this pattern in CCCL, see: https://github.com/NVIDIA/cccl/blob/main/CMakeLists.txt#L10-L14

By making this change (and adding a top-level CMakeLists.txt file to the NVTX repo per the suggestion above), CPM usage would be simplified to:

CPMAddPackage("gh:NVIDIA/NVTX@release-v3")

for most users. Currently, we must do

CPMAddPackage(
  NAME NVTX
  GITHUB_REPOSITORY NVIDIA/NVTX
  GIT_TAG release-v3
  DOWNLOAD_ONLY
  SYSTEM
)
include("${NVTX_SOURCE_DIR}/c/nvtxImportedTargets.cmake")

for the common case of wanting imported targets.

[alliepiper]

I'd be willing to push a PR with these changes if they sound agreeable to the NVTX team.