xapian-bindings: Fix build with slibtool

With slibtool the --config argument works differently than in GNU
libtool which results in 'auto/Xapian/Xapian$(PERL_SO)' failing to be
created. This can be worked around by using libtool --mode=install where
slibtool can copy the module to the intended location.

However GNU libtool still requires using --config otherwise it fails
during --mode=install with the following error.

  /bin/bash ../libtool --mode=install cp Xapian.la /home/runner/work/xapian/xapian/xapian-bindings/perl/auto/Xapian/Xapian.la
  libtool:   error: error: cannot install 'Xapian.la' to a directory not ending in /usr/local/lib/x86_64-linux-gnu/perl/5.26.1/auto/Xapian

gentoo issue: https://bugs.gentoo.org/793428

xapian-bindings/perl/Makefile.am

@@ -55,9 +55,14 @@ BUILT_SOURCES = Xapian.pm xapian_wrap.cc \
 
 auto/Xapian/Xapian$(PERL_SO): Xapian.la
 	@$(MKDIR_P) auto/Xapian
-	$(LIBTOOL) --config > libtoolconfig.tmp
+## GNU libtool and slibtool have different and incompatible usage for --config.
+	if $(LIBTOOL) --config > libtoolconfig.tmp 2>/dev/null; then \
 ## ksh requires a path on the sourced file.
-	. ./libtoolconfig.tmp; cp "$$objdir/Xapian$(PERL_SO)" auto/Xapian
+		. ./libtoolconfig.tmp; \
+		cp "$$objdir/Xapian$(PERL_SO)" auto/Xapian; \
+	else \
+		$(LIBTOOL) --mode=install cp $< $@; \
+	fi;
 	rm -f libtoolconfig.tmp
 
 EXTRA_DIST = perl.i except.i extra.i $(TESTS) \

xapian-bindings/python3/Makefile.am

@@ -88,9 +88,14 @@ xapian/__pycache__/__init__.@PYTHON3_CACHE_OPT1_EXT@: xapian/__init__.py xapian/
 
 xapian/_xapian$(PYTHON3_EXT_SUFFIX): _xapian.la
 	$(MKDIR_P) xapian
-	$(LIBTOOL) --config > libtoolconfig.tmp
+## GNU libtool and slibtool have different and incompatible usage for --config.
+	if $(LIBTOOL) --config > libtoolconfig.tmp 2>/dev/null; then \
 ## ksh requires a path on the sourced file.
-	. ./libtoolconfig.tmp; cp $$objdir/_xapian$(PYTHON3_EXT_SUFFIX) xapian
+		. ./libtoolconfig.tmp; \
+		cp $$objdir/_xapian$(PYTHON3_EXT_SUFFIX) xapian; \
+	else \
+		$(LIBTOOL) --mode=install cp $< $@; \
+	fi;
 	rm -f libtoolconfig.tmp
 
 CLEANFILES = \