libarchive: fix build error when linking with lzma

libarchive: fix build error when linking with lzma

Build fails when linking with liblzma compiled without threads support.

./.libs/libarchive.so: undefined reference to `lzma_cputhreads'
./.libs/libarchive.so: undefined reference to `lzma_stream_encoder_mt'

So let's enable LZMA only when toolchain has threads support.

Fixes:
http://autobuild.buildroot.org/results/2cb7f8a056982d0b894d0e2531ef357e74c8b796
http://autobuild.buildroot.org/results/d12954fbd460d396f255158fa127f39ff9d2be8d
http://autobuild.buildroot.org/results/abe10c70f38423a212ab27d7d27e22174eaa5aab
Many more...

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>