Attempt build without NNUE dependencies, if any

[ddugovic]

Conditionally build with or without NNUE support in an effort to remain compatible with http://www.variantfishtest.org:6543/ which supports Fairy-Stockfish (which does not support NNUE-related configuration or commands).

• Use NNUE defaults to true, so it would crash during search without a net
• Bench by default uses mixed mode, so same issue here
• Compilation requires more recent compiler versions than before, and we have not checked yet whether the current workers all fulfill those requirements.

https://github.com/ddugovic/Stockfish/commit/214b05ef52d6f0eb38f55f8670d253330cb17cff#commitcomment-41873056

[ddugovic]

I submitted a test which does not revert the compiler requirement (C++11 -> C++17) but does revert other NNUE changes.

[ddugovic]

Appveyor build is failing due to lack of -DUSE_NNUE in the dynamically generated CMakeLists.txt file: https://ci.appveyor.com/project/ddugovic/stockfish/builds/34999101