OK to use anonymous union (C11 feature)? Opinions wanted

[Ethan A M. <me...@uw...>]

Bug reports 2812 and 2813 report and diagnose a problem with color-handling
that manifests on ARM architectures but not on x86.  More detail is attached
to bug 2812 if you want gory details.

The basic issue is that different code is needed if color is represented by
a signed integer (e.g. "plot foo lt 2") or by an unsigned 32-bit ARGB value
(e.g. "splot foo with pm3d fillstyle transparent solid 0.25").

It is possible to modify the code by inserting an appropiate cast to either
   color.lt = (int)value     or    color.lt = (unsigned int)value
everywhere that it matters.

However IMO a cleaner approach is to modify the t_colorspec structure
using an anonymous union to distinguish between the two cases:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  typedef struct t_colorspec {
      colortype type;                 /* TC_<type> definitions below */
      union {
          int lt;                               /* used for TC_LT, TC_LINESTYLE */
          unsigned rgbcolor;      /* used for TC_RGB */
      };
      double value;                    /* used for TC_CB and TC_FRAC */
  } t_colorspec;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

That allows to write code like
    if (color.type == TC_RGB)
	color.rgbcolor = value;
    else
	color.lt = value;
I have a tested this out in a private git branch and it works as intended.

HOWEVER, anonymous unions were only added to the C standard in C11,
although both gcc and clang already supported them well before that.

So the question is, is it worth adding a requirement for C compiler that supports
C11 or at least supports anonymous unions?
As of gnuplot version 6 we're requiring c99 (version 5 was ok with c89).
MSVisualStudio claims to support c11 as of 2019.   I don't know what other
compilers people might be using, or what their level of support might be. 

What do you think?
Add it to the development version and back it out if compiler problems
are reported?   What about the stable version?  

	Ethan