libnetfilter_queue: automake portability warning

[Jeremy Sowden <jeremy@azazel.net>]

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Running autogen.sh gives the following output when it gets to
doxygen/Makefile.am:

  doxygen/Makefile.am:3: warning: shell find $(top_srcdir: non-POSIX variable name
  doxygen/Makefile.am:3: (probably a GNU make extension)

Automake doesn't understand the GNU make $(shell ...) function and tries
to interpret it as an Automake variable.  The Automake people would
probably say we shouldn't do that 'cause it's not portable:

  https://www.gnu.org/software/automake/manual/automake.html#Wildcards

However, if we accept that we are targetting GNU make, but we want to
get rid of the warning, I believe there are two ways to do so.  We can
tell Automake not to warn about non-portable constructions:

  diff --git a/configure.ac b/configure.ac
  index 4721eebbab1f..7cd34d079e67 100644
  --- a/configure.ac
  +++ b/configure.ac
  @@ -6,7 +6,7 @@ AC_CANONICAL_HOST
   AC_CONFIG_MACRO_DIR([m4])
   AC_CONFIG_HEADERS([config.h])

  -AM_INIT_AUTOMAKE([-Wall foreign subdir-objects
  +AM_INIT_AUTOMAKE([-Wall -Wno-portability foreign subdir-objects
          tar-pax no-dist-gzip dist-bzip2 1.6])
   m4_ifdef([AM_PROG_AR], [AM_PROG_AR])

On the other hand, if we want to suppress the warning for just this one
GNU-ism, we can hide it from automake:

  diff --git a/configure.ac b/configure.ac
  index 4721eebbab1f..b2b54d3168ad 100644
  --- a/configure.ac
  +++ b/configure.ac
  @@ -56,6 +56,19 @@ AS_IF([test "x$DOXYGEN" = x], [
                  with_doxygen=no
          ])
   ])
  +#
  +# Putting $(shell ... ) directly into the doyxgen Makefile.am confuses automake,
  +# which tries to interpret it as an automake variable:
  +#
  +#   doxygen/Makefile.am:3: warning: shell find $(top_srcdir: non-POSIX variable name
  +#   doxygen/Makefile.am:3: (probably a GNU make extension)
  +#
  +# Instead, we use autoconf to substitute it into place after automake has run.
  +#
  +AS_IF([test "x$with_doxygen" != no], [
  +  AC_SUBST([DOC_SRCS], ['$(shell find $(top_srcdir)/src -name '"'"'*.c'"'"')'])
  +])
  +
   AC_OUTPUT

   echo "
  diff --git a/doxygen/Makefile.am b/doxygen/Makefile.am
  index f38009b24114..6ed30e21ff75 100644
  --- a/doxygen/Makefile.am
  +++ b/doxygen/Makefile.am
  @@ -1,6 +1,6 @@
   if HAVE_DOXYGEN

  -doc_srcs = $(shell find $(top_srcdir)/src -name '*.c')
  +doc_srcs = @DOC_SRCS@

   doxyfile.stamp: $(doc_srcs) Makefile.am
          rm -rf html man

J.

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