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/Makefile b/src/Makefile
index bcfbbb7..ddb61dc 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1408,6 +1408,11 @@
 
 SHELL = /bin/sh
 
+# We would normally use "mkdir -p" but it doesn't work properly everywhere.
+# Using AC_PROG_MKDIR_P in configure.ac has a problem with the "auto"
+# directory.  Always use the install-sh script, it's slower but reliable.
+MKDIR_P = $(SHELL) install-sh -c -d
+
 .SUFFIXES:
 .SUFFIXES: .c .o .pro