Add -I m4 to ACLOCAL_AMFLAGS

We put m4 files into a separate directory. If we ever put our own files in
there, we need to include it. Furthermore, this suppresses annoying
autotools warnings.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Remove deprecated SYMBOL_CACHE_SIZE from doxygen.in

SYMBOL_CACHE_SIZE was removed in newer doxygen releases. We use the
default value, so simply drop it.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>