- Implement a minimal autotools build for libgumbo.
- Make `rake gumbo:test` use the same libgumbo.a used to build the
  nokogiri extension.
- Make changes to `gumbo-parser/src` trigger a rebuild of libgumbo.

- remove the `rebuild-libgumbo` target which is probably not needed
  once #3220 is merged
- avoid downloading googletest when just running `rake compile`
- update the Manifest check to ignore new gumbo-parser/ files
- simplify the 'host' variable in gumbo.rake, since we're only using
  it in development (and not cross-compiling)
- put back the libgumbo $libs, $LIBPATH, and include flags modifications
- make sure libgumbo is built static

and make sure autoconf is installed in the basic ubuntu container in CI