btrfs-progs: docs: fix build due to phony contents.rst

[BUG]
Since commit 8049446 ("btrfs-progs: docs: placeholder for
contents.rst file on older sphinx version"), on systems with much newer
sphinx-build, "make" would not work for Documentation directory:

  $ make clean-all && ./autogen.sh && ./configure --prefix=/usr/ && make -j12
  $ ls -alh Documentation/_build
  ls: cannot access 'Documentation/_build': No such file or directory

The sphinx-build has a much newer version:

  $ sphinx-build --version
  sphinx-build 7.2.6

[CAUSE]
On systems which don't need the workaround, the phony target of
contents.rst seems to cause a dependency loop:

  GNU Make 4.4.1
  Built for x86_64-pc-linux-gnu
  Copyright (C) 1988-2023 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.
  Reading makefiles...
  Reading makefile 'Makefile'...
  Updating makefiles....
   Considering target file 'Makefile'.
    Looking for an implicit rule for 'Makefile'.
     Trying pattern rule '%:' with stem 'Makefile'.
    Found implicit rule '%:' for 'Makefile'.
   Finished prerequisites of target file 'Makefile'.
   No need to remake target 'Makefile'.
  Updating goal targets....
  Considering target file 'contents.rst'.
   File 'contents.rst' does not exist.
  Finished prerequisites of target file 'contents.rst'.
  Must remake target 'contents.rst'.
  Makefile:35: update target 'contents.rst' due to: target is .PHONY
  if [ "$(sphinx-build --version | cut -d' ' -f2)" \< "1.7.7" ]; then \
  	touch contents.rst; \
  fi
  Putting child 0x64ee81960130 (contents.rst) PID 66833 on the chain.
  Live child 0x64ee81960130 (contents.rst) PID 66833
  Reaping winning child 0x64ee81960130 PID 66833
  Removing child 0x64ee81960130 PID 66833 from chain.
  Successfully remade target file 'contents.rst'.

All the default make doing is just try to generate contents.rst, but
since we have much newer version, we won't generate the file at all.

[FIX]
Instead of a phony target, just move the contents.rst generation into
man page target so that we won't cause loop target on contents.rst.

Fixes: 8049446 ("btrfs-progs: docs: placeholder for contents.rst file on older sphinx version")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

Documentation/Makefile.in

@@ -28,19 +28,16 @@ man5dir = $(mandir)/man5
 man8dir = $(mandir)/man8
 
 .PHONY: all man help
-.PHONY: contents.rst
+
+# Build manual pages by default
+all: man
 
 # Workaround for old sphinx that requires the contents.rst file
-contents.rst:
+man:
 	@if [ "$$(sphinx-build --version | cut -d' ' -f2)" \< "1.7.7" ]; then \
 		touch contents.rst; \
 	fi
 
-# Build manual pages by default
-
-all: man
-
-man:
 	$(QUIET_SPHINX)$(SPHINXBUILD) -M man "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
 help: