rtorrent: stop discarding the target build flags

rtorrent: stop discarding the target build flags

CONFIGURE_VARS is assigned rather than appended to, which discards the
default from package-defaults.mk: the cross compiler settings and the
CFLAGS, CXXFLAGS, CPPFLAGS and LDFLAGS the build system passes. The
client is configured with autoconf's own -g -O2 and built for the
toolchain's default architecture instead, without the target CPU flags,
the hardening flags and -ffile-prefix-map.

Fixes: 6d9d87166017 ("rtorrent: update to 0.15.3")
Signed-off-by: Daniel Golle <redacted>