fix size_t/uint64_t mismatch
quash warning
fix warnings
Update INSTALL

Update the INSTALL file:
- Remove ffmpeg from the list of external libraries.
- Change libmagic licence type to "BSD" (from "GPL"). I downloaded
  file-5.13.tar.gz from the official download location, and its
  COPYING file clearly contains a (2-clause) BSD licence. I also
  verified that Debian (libmagic1 package) lists it as BSD too, just
  to make sure.
- Update minimum required version for autoconf.