doc: fix macOS environment variables for ccache

macOS requires `cc` and `c++` rather than `gcc` and `g++`.

Closes: #40542

PR-URL: #40550
Fixes: #40542
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

BUILDING.md

@@ -519,6 +519,8 @@ $ make test-only
 If you plan to frequently rebuild Node.js, especially if using several branches,
 installing `ccache` can help to greatly reduce build times. Set up with:
 
+On GNU/Linux:
+
 ```console
 $ sudo apt install ccache   # for Debian/Ubuntu, included in most Linux distros
 $ ccache -o cache_dir=<tmp_dir>
@@ -527,6 +529,16 @@ $ export CC="ccache gcc"    # add to your .profile
 $ export CXX="ccache g++"   # add to your .profile
 ```
 
+On macOS:
+
+```console
+$ brew install ccache      # see https://brew.sh
+$ ccache -o cache_dir=<tmp_dir>
+$ ccache -o max_size=5.0G
+$ export CC="ccache cc"    # add to ~/.zshrc or other shell config file
+$ export CXX="ccache c++"  # add to ~/.zshrc or other shell config file
+```
+
 This will allow for near-instantaneous rebuilds even when switching branches.
 
 When modifying only the JS layer in `lib`, it is possible to externally load it