docs: Add some notes on building for macOS

I intend this page to grow as I explore more about Mesa/Darwin.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10152>

docs/index.rst

@@ -109,6 +109,7 @@ and other operating systems.
    dispatch
    gallium/index
    android
+   macos
    Linux Kernel Drivers <https://www.kernel.org/doc/html/latest/gpu/>
 
 .. toctree::

docs/macos.rst

@@ -0,0 +1,17 @@
+Notes for macOS
+================
+
+Mesa builds on macOS without modifications. However, there are some details to
+be aware of.
+
+-  Mesa has a number of build-time dependencies. Most dependencies, including
+   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.
+
+At the moment, only the software rasterizers are supported on macOS. Stay tuned
+for updates here!