docs/macos: Update for recent Mesa changes

- Default c_std is now c11, no need to workaround
  89b4f337d5 ("c_std=c11 in meson default_options")

- gallium-xlib has been renamed to xlib:
  76791db088 ("mesa/x11: Remove the swrast-classic-based fake libGL")

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14216>

docs/macos.rst

@@ -8,8 +8,6 @@ be aware of.
    Meson itself, are available in `homebrew <https://brew.sh>`, which has a
    Mesa package for reference. The exception seems to be Mako, a Python module
    used for templating, which you can install as `pip3 install mako`.
--  macOS's default C compiler doesn't play nice with some C11 idioms used in
-   Mesa. To workaround, set `-Dc_std=c11`.
 -  macOS is picky about its build-time environment. Type `brew sh` before
    building to get the Homebrew dependencies in your path.
 
@@ -18,7 +16,7 @@ hardware-accelerated system OpenGL framework, to provide hardware acceleration
 to X11 applications on macOS running via XQuartz.
 
 Mesa's software rasterizers also work on macOS. To build, set the build options
-`-Dosmesa=true -Dglx=gallium-xlib` and select an appropriate Gallium software
+`-Dosmesa=true -Dglx=xlib` and select an appropriate Gallium software
 rasterizer.
 
 For native hardware, stay tuned for updates here!