Add literal operator "" for C++11 support.
Allow u8 strings and user-defined literal suffixes for C++11.

This patch does not provide support for all C++11 literals.
1) it doesn't support "L", "u", or "U" strings because flex's regexps
expect 8-bit strings, so special code will be needed.
2) it doesn't support raw strings, because special code would be needed.
3) supporting wchar_t would require knowing what compiler (and what compiler
settings) are being emulated, because it's size is implementation-dependent.

The code in vtkParsePreprocess.c does not understand prefixes or suffixes
on strings, so u8 strings or strings with user defined suffixes may not
appear in preprocessor directives, except within the body of a #define.