dev-db/tarantool: allow to build against system libraries (xxhash, nghttp2, libunwind)

[Totktonada]

The libraries of the question:

• dev-libs/xxhash
  • Problem solved by bundling: build: linking failure when building with external ZSTD tarantool#6135
  • Patchset: Fix building with external zstd tarantool#6145
  • Where bundled: in 2.9.0-102-gce382f967 and newer
  • Where released as bundled: 2.10.0 and newer
• net-libs/nghttp2
  • Problem solved by bundling: Support HTTP/2 for http.client tarantool#5771
  • Patchset: build: link bundled libcurl with nghttp2 tarantool#6679
  • Where bundled: 2.10.0-beta1-378-gfa8d70caa and newer
  • Where released as bundled: 2.10.0 and newer
• sys-libs/libunwind
  • Problems solved using bundling:
    • Include fiber creation stack in fiber.info() tarantool#4002
    • enable backtrace at CentOS 8 tarantool#4611
  • Patchset: libunwind: add new submodule tarantool#6877
  • Follow ups: libunwind submodule follow-up fixes tarantool#6998
  • Where bundled: 2.10.0-beta2-177-g7dc9fe44a and newer
  • Where released as bundled: 2.10.0 and newer

We should let a user decide, whether to use system libraries (and take risks) or use ones shipped with tarantool. Set system ones as default if there are no known problems with versions offered by Gentoo.