Flex size_t sizes

Hi Akim,

Saw you have edited Flex, so I take it up here, even though not strictly a Bison topic:

The Apple flex version has been edited to admit size_t sizes, 64-bit on the platform, and perhaps it might be good idea for regular flex, which uses int, only 32-bit there. If using '%option c++' and mixing the versions, then the FlexLexer.h header is incompatible with the C++ source code, generating a compile error.

On MacOS with MacPorts and Xcode, the files are in
/opt/local/include/FlexLexer.h
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/FlexLexer.h