Update build.rst for C++17

[timsnyder-siv]

https://github.com/Xilinx/XRT/blame/master/src/runtime_src/doc/toc/build.rst#L16-L35

I think you guys require C++17 based on the fact that I'm getting messages like this from cmake:

CMake Error in /home/centos/XRT/tests/validate/ps_aie_test/CMakeLists.txt:
  Target "ps_aie.exe" requires the language dialect "CXX17" (with compiler
  extensions), but CMake does not know the compile flags to use to enable it.

I'm able to get cmake to generate after activating devtoolset-9 but I thought you might want to update your docs to reflect your C++ requirement.

[stsoe]

@uday, the documentation here is out-of-date: https://xilinx.github.io/XRT/master/html/build.html Will you update?