rollup merge of rust-lang#20380: dcrewi/fix-make-install

There seems to be a problem introduced by
8b3c676 that causes "make install"
to fail when the build is not configured to skip doc building.

Author: alexcrichton

mk/install.mk

@@ -28,7 +28,11 @@ endif
 # Remove tmp files because it's a decent amount of disk space
 	$(Q)rm -R tmp/dist
 
+ifeq ($(CFG_DISABLE_DOCS),)
+prepare_install: dist/$(PKG_NAME)-$(CFG_BUILD).tar.gz dist/$(DOC_PKG_NAME)-$(CFG_BUILD).tar.gz | tmp/empty_dir
+else
 prepare_install: dist/$(PKG_NAME)-$(CFG_BUILD).tar.gz | tmp/empty_dir
+endif
 
 uninstall:
 ifeq (root user, $(USER) $(patsubst %,user,$(SUDO_USER)))