doc: fix long lines in README.md
gitignore: ignore automake artifacts
build: unversion auto-generated build artifacts
build: fix up `make check` include path

Add the tests/ directory itself to the list of include paths.
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.