Fix plperl build

The changes in 639928c9 turned out to
require Perl 5.9.3, which is newer than our minimum required version.
So revert back to the old code for the normal case and only use the new
variant when both coverage and vpath are used.  As the minimum Perl
version moves forward, we can drop the old code sometime.

src/pl/plperl/GNUmakefile

@@ -83,7 +83,12 @@ all: all-lib
 
 %.c: %.xs
 	@if [ x"$(perl_privlibexp)" = x"" ]; then echo "configure switch --with-perl was not specified."; exit 1; fi
+# xsubpp -output option is required for coverage+vpath, but requires Perl 5.9.3
+ifeq ($(enable_coverage)$(vpath_build),yesyes)
 	$(PERL) $(XSUBPPDIR)/ExtUtils/xsubpp -typemap $(perl_privlibexp)/ExtUtils/typemap -output $@ $<
+else
+	$(PERL) $(XSUBPPDIR)/ExtUtils/xsubpp -typemap $(perl_privlibexp)/ExtUtils/typemap $< >$@
+endif
 
 
 install: all install-lib install-data