build: unversion auto-generated build artifacts

Don't version generated files, have autogen.sh create them at build
time. This should make life easier for people that have to re-run
autoconf (or automake) for their system, then end up with a git
index full of clobbered autotools boilerplate.

When I say 'people', I mean 'me.'
build: fix up `make check` include path

* Add the tests/ directory itself to the list of include paths.
* .gitignore the produced gumbo_test binary.
test: link against system libgtest if available

Make ./configure check if libgtest is available system-wide and if so,
link against that. Before this commit, the `make check` target required
a checkout of libgtest in $(srcdir)/gtest - somewhat inconvenient.