scons: add support for MAJOR_IN_{MKDEV,SYSMACROS}

src/gallium/winsys/svga/drm/vmw_screen.c: In function ‘vmw_dev_compare’:
src/gallium/winsys/svga/drm/vmw_screen.c:48:12: warning: implicit declaration of function ‘major’ [-Wimplicit-function-declaration]
   48 |    return (major(*(dev_t *)key1) == major(*(dev_t *)key2) &&
      |            ^~~~~
src/gallium/winsys/svga/drm/vmw_screen.c:49:12: warning: implicit declaration of function ‘minor’ [-Wimplicit-function-declaration]
   49 |            minor(*(dev_t *)key1) == minor(*(dev_t *)key2)) ? 0 : 1;
      |            ^~~~~

That file (and many others) already has the proper #include with their respective guards, but scons wasn't defining them, resulting in implicit functions being used instead (and an always-true check that's probably breaking something down the line).

Note that I'm cheating a bit here because Scons doesn't seem to have a clean way to detect the existence of major() et al. as functions or macros, so I'm taking the shortcut of just detecting the presence of the header and assuming its contents is what we expect.

/cc @jrfonseca