[PATCH bpf-next v2] bpftool: fix build failure

When building bpf selftests like
  make -C tools/testing/selftests/bpf -j20
I hit the following errors:
  ...
  GEN      /net-next/tools/testing/selftests/bpf/tools/build/bpftool/Documentation/bpftool-gen.8
  <stdin>:75: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
  <stdin>:71: (WARNING/2) Literal block ends without a blank line; unexpected unindent.
  <stdin>:85: (WARNING/2) Literal block ends without a blank line; unexpected unindent.
  <stdin>:57: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
  <stdin>:66: (WARNING/2) Literal block ends without a blank line; unexpected unindent.
  <stdin>:109: (WARNING/2) Literal block ends without a blank line; unexpected unindent.
  <stdin>:175: (WARNING/2) Literal block ends without a blank line; unexpected unindent.
  <stdin>:273: (WARNING/2) Literal block ends without a blank line; unexpected unindent.
  make[1]: *** [/net-next/tools/testing/selftests/bpf/tools/build/bpftool/Documentation/bpftool-perf.8] Error 12
  make[1]: *** Waiting for unfinished jobs....
  make[1]: *** [/net-next/tools/testing/selftests/bpf/tools/build/bpftool/Documentation/bpftool-iter.8] Error 12
  make[1]: *** [/net-next/tools/testing/selftests/bpf/tools/build/bpftool/Documentation/bpftool-struct_ops.8] Error 12
  ...

I am using:
  -bash-4.4$ rst2man --version
  rst2man (Docutils 0.11 [repository], Python 2.7.5, on linux2)
  -bash-4.4$

The Makefile generated final .rst file (e.g., bpftool-cgroup.rst) looks like
  ...
      ID       AttachType      AttachFlags     Name
  \n SEE ALSO\n========\n\t**bpf**\ (2),\n\t**bpf-helpers**\
  (7),\n\t**bpftool**\ (8),\n\t**bpftool-btf**\
  (8),\n\t**bpftool-feature**\ (8),\n\t**bpftool-gen**\
  (8),\n\t**bpftool-iter**\ (8),\n\t**bpftool-link**\
  (8),\n\t**bpftool-map**\ (8),\n\t**bpftool-net**\
  (8),\n\t**bpftool-perf**\ (8),\n\t**bpftool-prog**\
  (8),\n\t**bpftool-struct_ops**\ (8)\n

The rst2man generated .8 file looks like
Literal block ends without a blank line; unexpected unindent.
 .sp
 n SEEALSOn========nt**bpf**(2),nt**bpf\-helpers**(7),nt**bpftool**(8),nt**bpftool\-btf**(8),nt**
 bpftool\-feature**(8),nt**bpftool\-gen**(8),nt**bpftool\-iter**(8),nt**bpftool\-link**(8),nt**
 bpftool\-map**(8),nt**bpftool\-net**(8),nt**bpftool\-perf**(8),nt**bpftool\-prog**(8),nt**
 bpftool\-struct_ops**(8)n

Looks like that particular version of rst2man prefers to have actual new line
instead of \n.

Since `echo -e` may not be available in some environment, let us use `printf`.
Some comments are added in Makefile to warn that '%' is not allowed in bpftool
man page names.

Cc: Quentin Monnet <redacted>
Cc: Andrii Nakryiko <redacted>
Fixes: 18841da98100 ("tools: bpftool: Automate generation for "SEE ALSO" sections in man pages")
Signed-off-by: Yonghong Song <redacted>
---
 tools/bpf/bpftool/Documentation/Makefile | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/Documentation/Makefile b/tools/bpf/bpftool/Documentation/Makefile
index 4c9dd1e45244..5dd68d79671e 100644
--- a/tools/bpf/bpftool/Documentation/Makefile
+++ b/tools/bpf/bpftool/Documentation/Makefile

@@ -40,11 +40,13 @@ see_also = $(subst " ",, \
        $(foreach page,$(call list_pages,$(1)),",\n\t**$(page)**\\ (8)") \
        "\n")

+# using printf for portability as `echo -e` does not work in some
+# environments. Note that bpftool man page names should not include '%'.
 $(OUTPUT)%.8: %.rst
 ifndef RST2MAN_DEP
        $(error "rst2man not found, but required to generate man pages")
 endif
-       $(QUIET_GEN)( cat $< ; echo -n $(call see_also,$<) ) | rst2man $(RST2MAN_OPTS) > $@
+       $(QUIET_GEN)( cat $< ; printf $(call see_also,$<) ) | rst2man $(RST2MAN_OPTS) > $@