win32/@makefiles: only update "Extensions_static" if needed

Where needed is when: the file changes or any of the static
libraries are newer than Extensions_static

Previously it would always be updated, so perl5xx.dll would always
be rebuilt.

With gmake this prevents perl5xx.dll from being rebuilt.

For nmake it unfortunately doesn't, but may prevent updates in later
rebuilds.

MANIFEST

@@ -6477,6 +6477,7 @@ win32/perlhost.h			Perl "host" implementation
 win32/perllib.c				Win32 port
 win32/pod.mak				Win32 port
 win32/runperl.c				Win32 port
+win32/set_depend_modtime.pl		Win32: update target modtime to match deps
 win32/vdir.h				Perl "host" virtual directory manager for Win32
 win32/vmem.h				Perl "host" memory manager for Win32
 win32/win32.c				Win32 port

win32/GNUmakefile

@@ -1590,7 +1590,7 @@ Exts_static_general : ..\make_ext.pl $(CONFIGPM) Extension_lib $(GLOBEXE) $(HAVE
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(PLMAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --static !Unicode/Normalize
 
 Extensions_static : list_static_libs.pl Exts_static_general $(NORMALIZE_STATIC)
-	$(MINIPERL) -I..\lib list_static_libs.pl > Extensions_static
+	$(MINIPERL) -I..\lib list_static_libs.pl -o Extensions_static
 
 Extensions_nonxs : ..\make_ext.pl ..\pod\perlfunc.pod $(CONFIGPM) $(GLOBEXE)
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(PLMAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --nonxs !libs

win32/Makefile

@@ -1122,7 +1122,7 @@ Extensions_reonly: ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM
 Extensions_static : ..\make_ext.pl ..\lib\buildcustomize.pl list_static_libs.pl $(PERLDEP) $(CONFIGPM) Extensions_nonxs
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --static
-	$(MINIPERL) -I..\lib list_static_libs.pl > Extensions_static
+	$(MINIPERL) -I..\lib list_static_libs.pl -o Extensions_static
 
 Extensions_nonxs: ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM) ..\pod\perlfunc.pod
 	$(XCOPY) ..\*.h $(COREDIR)\*.*

win32/list_static_libs.pl

@@ -5,6 +5,12 @@
 
 use Config;
 
+my $out;
+if (@ARGV > 1 && $ARGV[0] eq "-o") {
+    shift;
+    $out = shift;
+}
+
 my @statics = split /\s+/, $Config{static_ext};
 
 my (@extralibs, %extralibs); # collect extralibs, preserving their order
@@ -13,5 +19,49 @@
     open my $fh, '<', $file or die "can't open $file for reading: $!";
     push @extralibs, grep {!$extralibs{$_}++} grep {/\S/} split /\s+/, join '', <$fh>;
 }
-print map {s|/|\\|g;m|([^\\]+)$|;"..\\lib\\auto\\$_\\$1$Config{_a} "} @statics;
-print map {"$_ "} @extralibs;
+
+my @libnames = join " ",
+    map {s|/|\\|g;m|([^\\]+)$|;"..\\lib\\auto\\$_\\$1$Config{_a}"} @statics,
+    @extralibs;
+my $result = join " ", @libnames;
+
+if ($out) {
+    my $do_update = 0;
+    # only write if:
+    #  - output doesn't exist
+    #  - there's a change in the content of the file
+    #  - one of the generated static libs is newer than the file
+    my $out_mtime;
+    if (open my $fh, "<", $out) {
+        $out_mtime = (stat $fh)[9];
+        my $current = do { local $/; <$fh> };
+        if ($current ne $result) {
+            ++$do_update;
+        }
+        close $fh;
+    }
+    else {
+        ++$do_update;
+    }
+    if (!$do_update && $out_mtime) {
+        for my $lib (@libnames) {
+            if ((stat $lib)[9] > $out_mtime) {
+                ++$do_update;
+                last;
+            }
+        }
+    }
+
+    unless ($do_update) {
+        print "$0: No changes, no libraries changed, nothing to do\n";
+        exit;
+    }
+
+    open my $fh, ">", $out
+        or die "Cannot create $out: $!";
+    print $fh $result;
+    close $fh or die "Failed to close $out: $!\n";
+}
+else {
+    print $result;
+}