libtorrent: stop discarding the target build flags

libtorrent: 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
library 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. On x86/geode that also costs
the 64-bit atomics: at the default -march=i486 the compiler cannot
inline them and emits __atomic_load_8 and __atomic_store_8 calls, so
every binary linking the static library needs libatomic.

Fixes: 13a058bed6c0 ("libtorrent: update to 0.15.3")
Signed-off-by: Daniel Golle <redacted>