my new clang-tidy version comes with some quite restrictive checks that fail on
current code in PQClean.

I have disabled the following three:
https://releases.llvm.org/11.0.0/tools/clang/tools/extra/docs/clang-tidy/checks/llvmlibc-restrict-system-libc-headers.html
https://clang.llvm.org/extra/clang-tidy/checks/cppcoreguidelines-init-variables.html
https://clang.llvm.org/extra/clang-tidy/checks/bugprone-reserved-identifier.html

Oussama Danba caught a bug in how we were handing a test related to
ciphertext encoding. Not a security issue as far as we can tell, but
certainly not the correct behavior.

More detail here: e0ab9525f1

This commit also switches ntruhrss701/avx2/poly_S3_inv to the
Bernstein--Yang code, also slightly simplifies the poly_Rq_to_S3
routine.

* Fix a bunch of issues in SPHINCS+ on Windows
* Fix testing makefile dependencies by fixing times of common files
  (affected Windows testing mainly)