libdht: give the shared library a soname

libdht: give the shared library a soname

The library installed as a bare libdht.so with no SONAME, so consumers
recorded the filename as their DT_NEEDED and there was no way to express
an ABI break. Build it as libdht.so.0 with a matching soname and declare
ABI_VERSION, so the package is versioned like any other shared library.

jech/dht makes no ABI promise of its own, so 0 is a packaged ABI epoch:
bump PKG_ABI_VERSION if the exported surface changes. That epoch is now
a single variable referenced from ABI_VERSION, the soname, the output
filename and the symlink target, rather than repeated literally in all
four places, matching the pattern already used by libcurl-gnutls.

Also pass TARGET_CFLAGS/TARGET_CPPFLAGS/TARGET_LDFLAGS to the compile
and link, which the hand-written rules had skipped, and fix
PKG_LICENSE_FILES: jech/dht ships LICENCE, not LICENSE, so the declared
file was never found.

Signed-off-by: Daniel Golle <redacted>