readlink giving unnecessary errors if stepmania is not yet installed

[jose1711]

Nothing critical, yet it would be nice to have this handled properly. First make invocation, stepmania not installed yet:

$ make
readlink: missing operand
Try 'readlink --help' for more information.
make system-prep
make[1]: Entering directory '/home/pi/raspbian-stepmania-arcade'
readlink: missing operand
Try 'readlink --help' for more information.
..

A quick fix to consider:

SM_INSTALL_DIR=$(shell dirname $$(readlink -f $$(which stepmania)) 2>/dev/null) => SM_INSTALL_DIR=$(shell dirname $$(readlink -f $$(which stepmania) 2>/dev/null) 2>/dev/null)