Use single quotes when there are backslashes in the filename

In the hope of unbreaking the buildfarm

src/tools/msvc/Solution.pm

@@ -276,14 +276,14 @@ s{PG_VERSION_STR "[^"]+"}{__STRINGIFY(x) #x\n#define __STRINGIFY2(z) __STRINGIFY
     if (IsNewer('src\include\utils\errcodes.h','src\backend\utils\errcodes.txt'))
     {
         print "Generating errcodes.h...\n";
-        system("perl src\backend\utils\generate-errcodes.pl src\backend\utils\errcodes.txt > src\backend\utils\errcodes.h");
+        system('perl src\backend\utils\generate-errcodes.pl src\backend\utils\errcodes.txt > src\backend\utils\errcodes.h');
         copyFile('src\backend\utils\errcodes.h','src\include\utils\errcodes.h');
     }
 
     if (IsNewer('src\pl\plpgsql\src\plerrcodes.h','src\backend\utils\errcodes.txt'))
     {
         print "Generating plerrcodes.h...\n";
-        system("perl src\pl\plpgsql\src\generate-plerrcodes.pl src\backend\utils\errcodes.txt > src\pl\plpgsql\src\plerrcodes.h");
+        system('perl src\pl\plpgsql\src\generate-plerrcodes.pl src\backend\utils\errcodes.txt > src\pl\plpgsql\src\plerrcodes.h');
     }
 
     if (IsNewer('src\interfaces\libpq\libpq.rc','src\interfaces\libpq\libpq.rc.in'))