NOTE: I'm checking this in just to have a record in source control of the idea.  We've decided for now it doesn't make sense to go forward with flag categories -- and the __VA_ARGS__ for macros has caused lots of problems with uncaught accidental commas, already -- so a future commit will back this out.  (Actually, it's all a series of commits, for annoying technical reasons.)

Add support for flag categories.

In this CL, all you can do is set categories in the DEFINE_*
macros and then retrieve them via GetCommandLineFlagInfo and
similar.

In future CLs, we will start to give some semantic meaning to
particular flag values, as described in the designdoc.  In
particular, we will start to use flag categories to revamp
--help output.

Implementation-wise: to keep categories an optional macro
argument, I had to use __VA_ARGS__, which means future gflags
releases will no longer work with MSVC 7.1.  We're at MSVC 10
now, so I'm pretty much ok with that.

The downside of __VA_ARGS__ is there is no error if you
specify more args after the ones we expect.  To get around
that, I only use __VA_ARGS_ in this idiom:
static const OptionalDefineArgs var = { __VA_ARGS__ };
The new OptionalDefineArgs struct defines all the args that
may be optionally specified in the DEFINE_* macros.  For now,
that's only the 'categories' arg, though in theory more could be
added later.

R=titus,ncalvin
DELTA=92  (54 added, 3 deleted, 35 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=3057


git-svn-id: https://gflags.googlecode.com/svn/trunk@61 6586e3c6-dcc4-952a-343f-ff74eb82781d