mkfontdir: Unexpanded ${exec_prefix}

Before commit 1f9d14be, path in mkfontdir was expanded.

Example with default invocation of configure:

#!/bin/sh

PATH="/usr/local/bin:$PATH"

exec mkfontscale -b -s -l "$@"

Since commit 1f9d14be, path in mkfontdir contains unexpanded ${exec_prefix}:

#!/bin/sh

PATH="${exec_prefix}/bin:$PATH"

exec mkfontscale -b -s -l "$@"

exec_prefix variable is set internally in configure and Makefile, but it is not expected to be set in normal shell environment.