configure finds mtx also at /usr/sbin/mtx

some distribution store mtx under /usr/sbin/mtx.
As configure runs as normal user, this path is not set.
This patch let configure also look at sbin paths.

fixes #101: variable MTX is not defined

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>

autoconf/configure.in

@@ -110,7 +110,7 @@ AC_PATH_PROG(CMP, cmp, [])
 AC_PATH_PROG(TBL, tbl, [])
 AC_PATH_PROG(AR, ar, [])
 AC_PATH_PROG(OPENSSL, openssl, [])
-AC_PATH_PROG(MTX, mtx, [])
+AC_PATH_PROG(MTX, mtx, [], [$PATH$PATH_SEPARATOR/usr/sbin$PATH_SEPARATOR/sbin])
 AC_PATH_PROG(DD, dd, [])
 AC_PATH_PROG(PYTHON, python, [])
 AC_PATH_PROG(PKGCONFIG, pkg-config, [])