patch 8.0.0576: can't build when configure choses "install-sh"

Problem:    Can't build when configure choses "install-sh". (Daniel Hahler)
Solution:   Always use install-sh.  Fix remaining use of mkinstalldirs.
            (closes #1647)
diff --git a/src/installman.sh b/src/installman.sh
index 0ac6c46..a2f09ad 100755
--- a/src/installman.sh
+++ b/src/installman.sh
@@ -39,7 +39,7 @@
 if test $what = "install" -o $what = "xxd"; then
    if test ! -d $destdir; then
       echo creating $destdir
-      ./mkinstalldirs $destdir
+      /bin/sh install-sh -c -d $destdir
    fi
 fi