apitrace: do not use bundled version of libpng

In case libpng is not found apitrace its own version of libpng. However
current bundled version (1.1.16) is impacted by
https://sourceforge.net/p/libpng/bugs/230/. We simplify problem by
forcing use of external libpng.

Notice this patch also impact use of zlib. Apitrace also use its own
version of zlib if none was found. Since we select libpng, external
zlib is also selected.

Fix:
  http://autobuild.buildroot.net/results/ac790191127cf4d8b19d0c7c640981264772f503/



Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>