Issue #15819: Fix out-of-tree builds from a readonly source.
diff --git a/configure.ac b/configure.ac
index 138d562..84d723a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -869,6 +869,7 @@
 	fi
 esac
 AC_PROG_INSTALL
+AC_PROG_MKDIR_P
 
 # Not every filesystem supports hard links
 AC_SUBST(LN)