Actually introduce the ability to build the Windows icons into the

Actually introduce the ability to build the Windows icons into the
Windows puzzle binaries. This checkin involves several distinct
changes:
 - mkfiles.pl now has an extra feature: if an object file is listed
   in Recipe with a trailing question mark, it will be considered
   optional, and silently dropped from the makefile if its primary
   source file isn't present at the time mkfiles.pl runs. This means
   people who check out the puzzles from Subversion and just run
   mkfiles.pl shouldn't get build failures; they just won't get the
   icons.
 - all the .R files now use this feature to include an optional
   Windows resource file.
 - the .rc resource source files are built by icons/Makefile.
 - windows.c finds the icon if present and uses it in place of the
   standard Windows application icon.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@7020 cda61777-01e9-0310-a592-d414129be87e