Overhaul C/CPP cmake configuration

[jrhemstad]

• Add a CMakeLists.txt to the c/ directoyr and create a nvtx3-c target for the NVTX C library
• Encapsulate the libdl dependency as a property of the nvtx3-c target
• Create a nvtx3-cpp target for the NVTX C++ library that depends on the nvtx3-c target
• Use CPM for Gbench/Gtest
• Clean up tests/benchmarks to depend on the nvtx3-cpp target
• Added the c/ directory as a symlink to cpp/nvtx-c to make it cleaner to depend on the nvtx-c cmake target
• Update to require C++14.

[jcohen-nvidia]

Looks good to me Jake. Feel free to merge unless you have some clever ideas for my comment above about preferring C++14 over C++11 if there's no explicit requirement for C++11.

[jcohen-nvidia]

OK, reviewed the new changes for requiring C++14. This looks good to merge.

[jrhemstad]

OK, reviewed the new changes for requiring C++14. This looks good to merge.

Great. Can you merge it? I don't have permissions to merge to the repo.

[jcohen-nvidia]

Is the git hash stuff ok now? If so I can merge this.

[kkraus14]

Is the git hash stuff ok now? If so I can merge this.

LGTM now, thanks!