Add pkg-config to the libpng usages as not all systems have the
symlink libpng.so -> libpng<version>.so (affected: NetBSD) that changes
with the version of the lib. If no-pkg-config is used, use -lpng as
before. Tested with FreeBSD 10.3, NetBSD 7.0.1 using png 1.6.21

Change-Id: I5c87f380c84da3d5c56c94da53adb900791c8caa
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>