xfsprogs: don't hard code the shell binary

xfsprogs: don't hard code the shell binary

Recent changes to debian unstable user space have caused the
xfsprogs build to break as certain shell functionality is being
assumed by libtool and friends.  The configure scripts test and
select the correct shell, but the input files ignore this and hard
code the shell to use and hence now break.

Fix this by using the shell that the configure scripts decide is the
right one to use.

Signed-off-by: Dave Chinner <dchinner@redhat.com>