configure: use proper quoting on iconv test
configure: use proper quoting on iconv test

Unquoted strings cause an error when the test variable is empty.

mwgoldsmith/libass,stps/libass,xbmc/libass,MrSmile/libass,libass/libass,Akemi/libass,xbmc/libass,astiob/libass,mwgoldsmith/libass,ShiftMediaProject/libass,grigorig/libass,grigorig/libass,Akemi/libass,ShiftMediaProject/libass,libass/libass,Akemi/libass,astiob/libass,mwgoldsmith/libass,MrSmile/libass,stps/libass,stps/libass