Made some modifications to the C++ lexer:

 * Modified FlexLexer.h so FlexLexer and yyFlexLexer are covered
   by the same #define, so both of them will only ever be defined
   once.

 * Copied testxxLexer.l in examples to frxxLexer.l. See initial
   comment in that file for building instructions

 * If you specify --yyclass to flex, a new class.yy.h file will be
   created if and only if it does not already exist. This class will
   derive from yyFlexLexer and have your scanner in its yylex.

 * If you're using more than one lexer in your program, you also need
   to specify -o class.yy.cc, or you're gonna have a bad time (But
   you were gonna have a bad time anyway.)

 * You'll need to #include your class.yy.h file in your .l if you
   specify --yyclass. I'll look into emitting that into the .cc file
   so you don't have to do that.

 * If you don't specify yyclass, FlexLexer and yyFlexLexer should
   function normally. For some value of normally. If you're doing something
   weird, it could break, and technically these days using flex
   qualifies as something weird.

 * You can add other methods to your myclass.yy.h file and call them
   from your .l file -- all the code in there looks like it gets called
   from the context of that class.

 TODOs:
 -Emit myclass #include into the .cc file so you don't have to
 -See if I can set up %option so you can specify the class name inside
  your .l file
 -Write a cppxml XML engine using flex (None of the rest of cppxml would
  have to change if I did this.)