Remove files which are created/altered by autoreconf -i

autoreconf has to be run anyway it seems. Also, document autoreconf in
INSTALL (and a minor note on --prefix confusion).
aclocal: warning: autoconf input should be named 'configure.ac', not 'configure.in'
config.h.in created on converting configure.ac→configure.in
Merge branch 'cleanup/autoreconf' of https://github.com/unhammer/gimp-sharp