package/libcurl: carefully override LD_LIBRARY_PATH

To build libcurl, we need to override LD_LIBRARY and force it to a sane
value, otherwise libcurl is confused when target == host (see a51ce319,
libcurl: fix configure with openssl when target == host).

That is currently OK, since we always set LD_LIBRARY_PATH to a non-empty
value.

However, we're soon to stop setting it at all.

So, if the user has an empty (or no) LD_LIBRARY_PATH in his envirnment,
we'd end up adding the current working directory to LD_LIBRARY_PATH (as
an empty entry in a colon-separated list is most probably interpreted as
meaning the current working directory, which we do know can cause issue,
and which we expressely check against in support/dependencies/dependencies.sh

Fix that by only using an existing LD_LIBRARY_PATH if it is not empty.
Also use a Makefile construct as it is easier to read than a shell one
(we can do that, as all variables from the environment are available as
make variables).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>