linux-doc: Copy source to build directory

This is necessary to avoid creating files in source directory, which
makes the linux-source package unclean (and unreproducible).

Firstly, Python creates bytecode files alongside the module sources.
We can and do exclude those, though.

Secondly, starting with 4.10, Documentation/media/Makefile converts
from dot to SVG and from SVG to PDF in the source directory.  These
can't easily be excluded, as SVG is also used as a source format.

debian/changelog

@@ -15,6 +15,8 @@ linux (4.10~rc6-1~exp1) UNRELEASED; urgency=medium
   * linux-doc: Update documentation file list for 4.10
   * linux-doc: Fix up symlinks to gzipped docs
   * debian/control: Add build-dependency on graphviz for documentation
+  * linux-doc: Copy source to build directory, to avoid creating files in
+    source directory
 
   [ Roger Shimizu ]
   * debian/copyright: Add GPL/X11 Dual License

debian/rules.real

@@ -192,10 +192,11 @@ $(STAMPS_DIR)/build_$(ARCH)_$(FEATURESET)_$(FLAVOUR):
 
 $(STAMPS_DIR)/build-doc: DIR=$(BUILD_DIR)/build-doc
 $(STAMPS_DIR)/build-doc: $(STAMPS_DIR)/source
-	rm -rf $(DIR)
-	mkdir -p $(DIR)
-	+$(MAKE_CLEAN) O='$(CURDIR)/$(DIR)' xmldocs
-	+$(MAKE_CLEAN) O='$(CURDIR)/$(DIR)' htmldocs mandocs
+	mkdir -p '$(BUILD_DIR)'
+	rm -rf '$(DIR)'
+	$(call copy_source,$(DIR))
+	+$(MAKE_CLEAN) -C '$(DIR)' xmldocs
+	+$(MAKE_CLEAN) -C '$(DIR)' htmldocs mandocs
 	@$(stamp)
 
 install-base: