configure: Remove the -dwarf2 argument for Mac OS X builds

Modern versions of Xcode properly support dwarf2, and as such dwarf2 is
always enabled. This change removes the ability to turn it off, making
dwarf2 non-optional.

Change-Id: I149daeae6048ee8a1ed116363572173ad219102e
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>