complibs/libelf: use host compiler

It appears, that the configure scripts of libelf versions 0.8.13 and
0.8.12 do not honour the --host option. The compiler must be given as an
environment variable or the process will use the command "gcc" as the
compiler.

It seems that this is already done in the function do_libelf_target in
scripts/build/companion_libs/libelf.sh, but not in function do_libelf.
(transplanted from 81b4c0efa46f757b89c150f66e09615ee7c0f5a4)

 COPYING