Replace some instances of "cmake -E copy_if_different" with a standalone script

@JapaMala reported that CMake < 3.5 doesn't support copy_if_different with
multiple source files.

https://cmake.org/cmake/help/v3.5/release/3.5.html#command-line

depends/copy-if-different.pl

@@ -0,0 +1,30 @@
+#!/usr/bin/perl
+
+# A replacement for "cmake -E copy_if_different" that supports multiple files,
+# which old cmake versions do not support
+
+# Usage: copy-if-different.pl src-file [src-file...] dest-dir
+
+use strict;
+use warnings;
+
+use Digest::SHA;
+use File::Basename;
+use File::Copy;
+
+sub sha_file {
+    my $filename = shift;
+    my $sha = Digest::SHA->new(256);
+    $sha->addfile($filename);
+    return $sha->hexdigest;
+}
+
+my $dest_dir = pop @ARGV or die "no destination dir";
+-d $dest_dir or die "not a directory: $dest_dir";
+my @src_files = @ARGV or die "no source files";
+
+foreach my $file (@src_files) {
+    my $dest = "$dest_dir/" . basename($file);
+    next if -f $dest && sha_file($file) eq sha_file($dest);
+    copy($file, $dest);
+}

library/CMakeLists.txt

@@ -239,7 +239,7 @@ ADD_CUSTOM_COMMAND(
     COMMAND protoc-bin -I=${CMAKE_CURRENT_SOURCE_DIR}/proto/
             --cpp_out=dllexport_decl=DFHACK_EXPORT:${CMAKE_CURRENT_SOURCE_DIR}/proto/tmp/
             ${PROJECT_PROTOS}
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different
+    COMMAND ${PERL_EXECUTABLE} ${CMAKE_SOURCE_DIR}/depends/copy-if-different.pl
             ${PROJECT_PROTO_TMP_FILES}
             ${CMAKE_CURRENT_SOURCE_DIR}/proto/
     COMMENT "Generating core protobufs"

plugins/CMakeLists.txt

@@ -64,7 +64,7 @@ ADD_CUSTOM_COMMAND(
                        -I=${CMAKE_CURRENT_SOURCE_DIR}/proto/
             --cpp_out=${CMAKE_CURRENT_SOURCE_DIR}/proto/tmp/
             ${PROJECT_PROTOS}
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different
+    COMMAND ${PERL_EXECUTABLE} ${CMAKE_SOURCE_DIR}/depends/copy-if-different.pl
             ${PROJECT_PROTO_TMP_FILES}
             ${CMAKE_CURRENT_SOURCE_DIR}/proto/
     COMMENT "Generating plugin protobufs"