debian: build with system jerasure and gf-complete

debian: build with system jerasure and gf-complete

Add libjerasure-dev to Build-Depends and enable WITH_SYSTEM_JERASURE
so that the Debian package builds ec_jerasure and ec_shec plugins
against the distribution-provided libraries instead of the vendored
copies.

libjerasure-dev pulls in libgf-complete-dev, so both libraries are
covered by a single build dependency.

Add libjerasure2 to ceph-base's Depends for the runtime shared
libraries. libjerasure2 in turn depends on libgf-complete1t64.
The explicit dependency is needed because dh_shlibdeps excludes
erasure-code plugins from automatic dependency scanning (they are
loaded via dlopen at runtime).

The distro-packaged jerasure and gf-complete are from the same
upstream git snapshots (2017/04/10) as ceph's vendored copies.
gf-complete source is byte-identical. jerasure differs only in
additional null-pointer checks and error handling in jerasure.c,
plus an extra galois_uninit_field() function that ceph does not use.
All function signatures ceph depends on are present and compatible.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>