Create doxgen.cfg dynamically

Create doxgen.cfg dynamically

The patch from bug#3799
	https://bugtrack.alsa-project.org/alsa-bug/view.php?id=3799

The "doc" target in doc/Makefile.am assumes the build directory and
the source directory are the same and fails if they differ ... this is
because the doxygen.cfg contains relative paths (such as ../) that
only work when building in the source tree

The attached patch against hg replaces all relative paths with
@top_srcdir@ and changes doxygen.cfg to a generated file so that
configure replaces @top_srcdir@ with the appropriate configure-time
path.

Assuming people like being able to do `make -f Makefile.am doc` on
an unconfigured and having it work, i added a small `test&&sed`
that'll generate an appropriate default doxygen.cfg for them.