build: Handle po4a --porefs difference between 0.58 and earlier versions

Before po4a 0.58 --porefs supported specifying a wrapping mode by
appending «,wrap» or «,nowrap», with the default being “nowrap”.
Starting with po4a 0.58 the default has changed to “wrap”, and these
modifiers are not accepted anymore, which causes a problem with
backwards compatibility as we cannot get consistent behavior across
different versions.

Detect this in configure and pass the «,wrap» modifier when needed
and supported, which should make it possible to build the man pages
again consistently in Debian buster and bullseye.

README

@@ -56,7 +56,7 @@ the source should be roughly equivalent to the distributed tar source.
 
 To enable translated documentation this software will be needed:
 
-  po4a >= 0.58
+  po4a >= 0.43 (>= 0.58 for addenda inclusion)
 
 
 Building from tar source

debian/changelog

@@ -67,7 +67,8 @@ dpkg (1.20.1) UNRELEASED; urgency=medium
     - Add debian/changelog as a Changes file to the CPAN distribution.
     - Set DPKG_SERIES to 1.20.x when running the functional test suite.
     - Change po4a --porefs argument from no longer supported “noline,wrap”
-      to “file”.
+      to “file,wrap” when the default contains «,nowrap» (po4a < 0.58), and
+      to “file” otherwise (po4a >= 0.58), to preserve backwards compatibility.
     - Switch to set the po4a addendum in the po4a_paths section instead of
       repeating it for each man page.
   * Packaging:

m4/dpkg-progs.m4

@@ -38,14 +38,28 @@ AC_DEFUN([DPKG_PROG_PERL], [
 # --------------
 AC_DEFUN([DPKG_PROG_PO4A], [
   AC_REQUIRE([AM_NLS])
+  AC_REQUIRE([DPKG_PROG_PERL])
   AC_ARG_VAR([PO4A], [po4a program])
   AC_CHECK_PROGS([PO4A], [po4a])
   AS_IF([test "$USE_NLS" = "yes" && test -n "$PO4A"], [
     USE_PO4A=yes
+    po4a_use_wrap=$($PERL -MLocale::Po4a::Po -e \
+                    'my $porefs = Locale::Po4a::Po->new()->{options}{porefs};
+                     print "yes" if $porefs =~ /,nowrap$/')
   ], [
     USE_PO4A=no
   ])
   AC_SUBST([USE_PO4A])
+
+  # Starting with po4a 0.58 the --porefs option does not accept the wrap
+  # modifiers, but earlier versions defaulted to nowrap, so we have to handle
+  # the backward incompatible change to support older and newer versions.
+  AS_IF([test "x$po4a_use_wrap" = "xyes"], [
+    PO4A_POREFS="file,wrap"
+  ], [
+    PO4A_POREFS="file"
+  ])
+  AC_SUBST([PO4A_POREFS])
 ])# DPKG_PROG_PO4A
 
 # DPKG_PROG_POD2MAN

man/Makefile.am

@@ -147,8 +147,9 @@ PO4A_V = $(PO4A_V_@AM_V@)
 PO4A_V_ = $(PO4A_V_@AM_DEFAULT_V@)
 PO4A_V_0 = @echo "  PO4A   $@";
 
+PO4A_POREFS = @PO4A_POREFS@
 PO4A_OPTS = --previous --srcdir $(srcdir) --destdir $(builddir) --no-backups \
-            --porefs file --msgmerge-opt=--add-location=file \
+            --porefs $(PO4A_POREFS) --msgmerge-opt=--add-location=file \
             --package-name dpkg-man --package-version $(PACKAGE_VERSION) \
             --copyright-holder "$(PACKAGE_COPYRIGHT_HOLDER)" \
             --msgid-bugs-address $(PACKAGE_BUGREPORT)