Makefile.am: Do not ship generated grammar files

The generated grammar files are tied to a particular syslog-ng version,
and are not reusable. The tarball should compile with all supported
versions of syslog-ng, regardless of grammar files.

For this reason, BUILT_SOURCES have been dropped, and nodist_* variables
are used instead. This also means that one needs flex & bison to compile
the Incubator, which is now noted in the README.

This fixes #5.

Reported-by: Peter Czanik <czanik@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
README.md: Update the syslog-ng github repo URL

The source repository has been moved to an unversioned repo some time
ago, so update the reference. While the versioned repository will
continue working, it is better to point people towards the canonical one
instead.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>