Consider using pragmas to always mark nvtx headers as system headers and silence warnings

[jrhemstad]

One of the differences between using -I vs -isystem is that headers found through -isystem will not emit warnings in user code. This is convenient when user code compiles with stricter warnings that might trigger a warning in nvtx header code, but it requires users to know to use -isystem (or a cmake SYSTEM include).

A more robust option that doesn't require people to know the difference between -I and -isystem is to use #pragma system_header on the top of every NVTX header to automatically make it as if the header were found via -isystem and silence any warnings.

For context, we did the same thing with CCCL headers here: NVIDIA/cccl#527

This would look something like:

// Check for GCC (probably needs version checks)
#if defined(__GNUC__) && !defined(__clang__)
    #define NVTX3_IMPLICIT_SYSTEM_HEADER _Pragma("GCC system_header")

// Check for Clang (probably needs version checks)
#elif defined(__clang__) 
    #define NVTX3_IMPLICIT_SYSTEM_HEADER _Pragma("clang system_header")

// Check for MSVC (probably needs version checks)
#elif defined(_MSC_VER) 
    #define NVTX3_IMPLICIT_SYSTEM_HEADER __pragma(system_header)

// If none of the known compilers are detected, define it as an empty statement
#else
    #define NVTX3_IMPLICIT_SYSTEM_HEADER
#endif

// If the disable flag is set, define it as an empty statement
#else
    #define NVTX3_IMPLICIT_SYSTEM_HEADER
#endif