Explicitly use SRCDIR and BUILDDIR in Doxyfile files to reference input
files. This seems to be a sufficient fix for an issue where the build
would fail when libsigrok/ was located in a directory that was a symlink.
This fixes bug #547.
This is more specific and prevents any potential issues e.g. when
multiple distro packages might ship with a generic file like
sigrok-logo-notext.png that's supposed to be installed in the same place.