doc: fix long lines in README.md
gitignore: ignore automake artifacts
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 and test runner logs.
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.
build: fix race condition in test deps

Declare proper transitive dependencies when building with an in-tree
copy of libgtest so that `make` knows when to build what.

A nice side effect is that it no longer runs libgtest's configure script
on every invocation of `make check`. Should speed up the edit/build/test
cycle a bit.

Newer versions of automake are more aggressive when it comes to
parallelizing builds and that becomes apparent when executing
`make -j<n> check` with n >= 2: the test suite build failed spuriously
because `make` tried to compile the test runner and run libgtest's
configure script at the same time.
Merge pull request #19 from bnoordhuis/assorted-autotools-fixups

Assorted autotools fixups