[misc] Set CMAKE_CXX_STANDARD to 17

[xumingkuan]

This is to let CLion recognize C++17 features... Without this line (set(CMAKE_CXX_STANDARD 17)), CLion keeps complaining about stuff like std::optional does not name a type.

See also https://stackoverflow.com/questions/53505844/clion-does-not-recognise-c17-features.

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[codecov]

Codecov Report

Merging #1964 into master will increase coverage by 0.94%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1964      +/-   ##
==========================================
+ Coverage   42.69%   43.64%   +0.94%     
==========================================
  Files          45       45              
  Lines        6436     6225     -211     
  Branches     1106     1106              
==========================================
- Hits         2748     2717      -31     
+ Misses       3513     3334     -179     
+ Partials      175      174       -1     

Impacted Files	Coverage Δ
python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
python/taichi/testing.py	75.00% <0.00%> (-0.72%)	⬇️
python/taichi/lang/linalg.py	89.33% <0.00%> (-0.67%)	⬇️
python/taichi/lang/meta.py	62.82% <0.00%> (-0.48%)	⬇️
python/taichi/lang/__init__.py	40.45% <0.00%> (-0.33%)	⬇️
python/taichi/misc/util.py	18.27% <0.00%> (-0.24%)	⬇️
python/taichi/misc/task.py	0.00% <0.00%> (ø)
python/taichi/tools/patterns.py	0.00% <0.00%> (ø)
python/taichi/lang/kernel.py	71.39% <0.00%> (+0.13%)	⬆️
python/taichi/misc/gui.py	8.89% <0.00%> (+0.18%)	⬆️
... and 9 more

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[yuanming-hu]

Cool! I guess it may make more sense to move this line to the very beginning of the root CMakeLists.txt, since this option applies to the entire project?

As a CLion user on Linux, I didn't run into the warning. It seems to be platform-specific.