Example show why cxxflags is necessary.

Define -std=c++17 in cxxflags, no complain from tinygo

tinygo build -target=./good.json -o test.wasm .

Define -std=c++17 in cflags, show error: invalid argument '-std=c++17' not allowed with 'C' error

tinygo build -target=./bad.json -o test.wasm .