[Unfortunately,] @top_srcdir@ is replaced by the package's top-level
directory relative to the directory of the .in file being processed,
e.g. if you unpack the MPD source to /usr/src/mpd, and build it in the
same directory, @top_srcdir@/src/ in doc/doxygen.conf.in will be re-
placed by "../src/", and, as doxygen is invoked from the top directory,
is will expect the source code in /usr/src/src/, which is obviously
wrong.
To work around this problem, this patch changes @top_srcdir@ to
@abs_top_srcdir@, which expands to the absolute path of the top source
directory[1].

[1] http://www.gnu.org/s/hello/manual/autoconf/Preset-Output-Variables.html