- moved slang shaders to spvshaders to separate software and hardware
  shaders
- split Makefile.am into Makefile.am, src/Makefile.am and
  src/spvshaders/Makefile.am
- build shaders as DATA primary, instead of both PROGRAMS and SCRIPTS,
  which greatly simplifies building. Before that, we had to first build
  them as PROGRAMS, and then copy them as SCRIPTS to remove executable
  extensions (i.e. ".exe" for Windows)
- changed final executable dir from enginedir to bindir, which makes
  automake see it as an exec instead of data
- compute relative paths from C++, which makes it possible to change
  dirs arbitrarily when running make while still having a relocatable
  executable
- updated README.md to reflect changes in dirs
- use AX_COMPARE_VERSION() to simplify checking if Vulkan headers
  version is correct
- factored out most of paths computation in engine::path_utils::Paths