Generate build system and remove autotools-related files from
.gitignore so that the user is not required to run autoreconf.

Mainly done for convenience, but also because some platforms
(OS X, notably) can have broken autotools environments and this
leads to either the autoreconf step failing outright or to the
generated build system being faulty in different ways.
Standardize all include headers with their absolute paths within the AvxSynth source tree. There are only two exceptions to this:

Installed headers, which refer to each other locally, since
they all get installed to <>/avxsynth/

Headers which do not exist in the source tree. These were left alone,
but now have comments disclaiming the fact that they don't exist
in the tree. This makes it easy to find them. In the event that
these functions/headers ever do need to be brought back, then they'll
need to be properly adjusted for the absolute path convention of
the other headers.
Synchronize avxffms2 with upstream r753