[RFC] Dont use a global build dir for third-party deps

- The third-party CMakeLists.txt defaulted to install in a shared .deps
  folder, it now uses its own binary dir, allowing for multiple third-party
  builds at the same time.
- Updated the Makefile wrapper to build dependencies using build/deps
- Neovim CMakelists.txt now uses ${CMAKE_BINARY_DIR}/deps instead
  of ${CMAKE_CURRENT_SOURCE_DIR}/.deps for dependencies (by default)
- Removed .deps from .gitignore

I wont argue too strongly about it since it is basically a trade off between:

- If we have a single .deps folder we get to share the dependencies with multiple builds, good for speed when using lots of builds.
- If we are testing changes to the dependencies then it helps being able to having several builds+deps with different flags. Or when doing builds for multiple architectures/OS at the same time (that can't share .deps).
- Personally I find it "cleaner" to use build/deps instead of .deps

For people just using the main Makefile to build once this should have no impact.