fail in configure if mtree is not present, rather than later in the build (see https://trac.macports.org/ticket/32531)

git-svn-id: https://svn.macports.org/repository/macports/trunk/base@91520 d073be05-634f-4543-b044-5fe20cf6d1d6

configure

@@ -5986,6 +5986,10 @@ fi
 
 
 
+if test "x$MTREE" = "x"; then
+    as_fn_error $? "mtree not found" "$LINENO" 5
+fi
+
 
 	# Extract the first word of "sed", so it can be a program name with args.
 set dummy sed; ac_word=$2

configure.ac

@@ -165,6 +165,10 @@ AC_ARG_VAR(TAR, [path to tar command])
 AC_ARG_VAR(XAR, [path to xar command])
 AC_ARG_VAR(XZ, [path to xz command])
 
+if test "x$MTREE" = "x"; then
+    AC_MSG_ERROR([mtree not found])
+fi
+
 MP_SED_EXTENDED_REGEXP
 MP_TAR_FAST_READ
 MP_TAR_NO_SAME_OWNER