Modernize Autotools usage, include real install instructions

The code has bitrotted somewhat since the last commit, and
frankly, Autotools has never been a friendly technology.
But this should clean up the way it handles on modern
systems, and give users a bit more guidance in setting up
this synth.

That's not to say it will be a guaranteed smooth ride, but
it never hurts to do a bit of dusting and sand the
sharpness off some edges.

# Please enter the commit message for your changes. Lines starting with '#' will be ignored, and an empty message
# aborts the commit. On branch master Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
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#	modified: README modified: configure.ac modified: gui/Makefile.am
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# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
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#	compile config.h.in~ gui/nekostring_gtk