CMake: Allow selection of C++ standard via cache variable.
At CGG, we take advantage of newer C++ versions in our plugins and usually build OF with C++17 or 20, accomplished by editing the CMakeLists.txt. This MR adds a convenient selection (dropdown in cmake-gui) to select the desired version.
In addition, disable CXX_EXTENSIONS
(nonportable things like -std=gnu++11
).