Switch MSVC to W4 rather than Wall

This avoids many spurious warnings from:
https://docs.microsoft.com/en-us/cpp/preprocessor/compiler-warnings-that-are-off-by-default
Bug:
Change-Id: Ic8e0e5fc894e3b78ba55c4825a49e904df3b49f2
1 file changed
tree: a3fe9e47efd2757a9f8fb5e16f3facdbf34f9604
  1. src/
  2. .gitignore
  3. build.ninja
  4. CMakeLists.txt
  5. LICENSE
  6. skcms.c
  7. skcms.h
  8. tests.c