libsdl-mixer: fix S!=B failure

This package does not like being built out-of-tree.

Fixes:
| configure: error: cannot find install-sh, install.sh, or shtool in
[...]

Signed-off-by: Tim Orling <TicoTimo@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>