* debian/rules
  - convert to dh style
  - set "export DEB_BUILD_MAINT_OPTIONS = hardening=+all,-pie" since
    avoid to build failure.
* debian/libsylph-dev.examples
  - move from rules
* debian/control
  - set "Build-Depends: debhelper (>= 9)"
  - add "Pre-Depends: ${misc:Pre-Depends}" for Multi-Arch.
  - add "Multi-Arch: same"
* debian/compat
  - set 9
* debian/libsylph{,-dev}.install
  - deal with Multi-Arch.