ltrace: disable elfutils unwinding when libunwind is used

ltrace can't use both. Use --with-elfutils=no to disable use of elfutils
(libdwfl) for unwinding.

Note that elfutils (libelf) is still a mandatory dependency regardless of
--with-elfutils.

Fixes:
http://autobuild.buildroot.net/results/a3c/a3c1647578cdeda3aaf672084a9a893f06dbad26/
http://autobuild.buildroot.net/results/fdb/fdb7e760faeda77b263a6dac3e322a25b53e67ff/
http://autobuild.buildroot.net/results/70a/70a0614ca7baf6a46898834a87cd46865f6e3385/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>