>	if [[ -z "$1" ]]; then

either lose the quotes or lose the double brackets.  hint: the former.

> 		echo "!!! xmms2_flag() called without a parameter." >&2
>		echo "!!! xmms2_flag() <USEFLAG> [<xmms2_flagname>]" >&2

use eerror or die

> 			if use "$1"; then

USE flag cannot contain spaces or anything else that requires quoting

> 			else
>				: # nothing is generated
>			fi

my, we sure do like to be verbose

> 			--destdir="${D}" \

you need to give the DESTDIR to configure ?  no wonder xmms2 sucks

>			${CTARGET:+--with-target-platform=${CTARGET}} \

i highly suspect this isnt what you think it is.  if this option is for cross-
compiling, then use ${CHOST}.

>			optionals+="$(xmms2_flag $option)"

dont need the quoting when doing ...=$(...)

>	"${S}"/waf

the default dir for most src_* funcs is $S, so just use ./waf

>	use python && python_mod_optimize $(python_get_sitedir)/xmmsclient

shouldnt sitedir be quoted ?
-mike