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elfutils: towards uClibc support: argp-standalone usage

elfutils uses the argp family of functions, that isn't available in
uClibc. So, we add a dependency on argp-standalone if building with
uClibc, and modify elfutils source code to link against argp if
needed.

Heavily based from work done by Stefan Fröberg, but with many further
modifications by Thomas Petazzoni.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>