Build all demos as C89

[ccawley2011]

This was originally submitted as vurtun/nuklear#814. It also supercedes PR #39.

[Hejsil]

LGTM, I only have a minor comment

[RobLoach]

Looks like there are conflicts popping up.

[dumblob]

We're sorry for the delay which apparently causes merge conflicts. Would you or anyone else be willing to fix the conflicts? We'll then do final skim through it and merge it. Thanks!

[ccawley2011]

We're sorry for the delay which apparently causes merge conflicts. Would you or anyone else be willing to fix the conflicts? We'll then do final skim through it and merge it. Thanks!

OK, done.

[RobLoach]

Was able to build all allegro, glfw, sdl, sfml, wayland, and x11 demos. Did find the following warnings when building x11_rawfb for nuklear_rafb.h, and the unused functions, but other than that, this looks good...

In file included from main.c:50:
nuklear_rawfb.h: In function ‘nk_rawfb_img_setpixel’:
nuklear_rawfb.h:182:27: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
  182 |         ptr = img->pixels + (img->pitch * y0);
      |                           ^
nuklear_rawfb.h: In function ‘nk_rawfb_img_getpixel’:
nuklear_rawfb.h:201:27: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
  201 |         ptr = img->pixels + (img->pitch * y0);
      |                           ^
In file included from main.c:49:
At top level:
../../nuklear.h:6166:1: warning: ‘nk_cos’ defined but not used [-Wunused-function]
 6166 | nk_cos(float x)
      | ^~~~~~
../../nuklear.h:6150:1: warning: ‘nk_sin’ defined but not used [-Wunused-function]
 6150 | nk_sin(float x)
      | ^~~~~~
../../nuklear.h:6136:1: warning: ‘nk_inv_sqrt’ defined but not used [-Wunused-function]
 6136 | nk_inv_sqrt(float n)
      | ^~~~~~~~~~~

[dumblob]

Thanks everyone! Especially @ccawley2011 for years-long patience!