elfutils: towards uClibc support: FTS functions

The fts_*() functions are optional in uClibc, and not compiled in our
default configuration. The best option would be to migrate this
elfutils code to the nftw family of functions, but it requires quite
some work.

So we have several options here:

 *) Enable fts_*() functions in our default uClibc configuration. Not
    nice since only one package needs them (the help text of uClibc
    for fts_*() functions explicitly mention that they have been added
    to be able to build elfutils).

 *) Use gnulib, but it is quite heavy to setup, requires modifications
    to configure.ac, and other things.

 *) Copy the fts function from uClibc into elfutils source code. This
    is the solution used below. uClibc is LGPL, and elfutils is
    LGPL/GPL, so there should not be any licensing issue.

Of course, the fts_*() functions are only built if they are not
already provided by the C library.

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