doc: fix environment variable settings for ccache

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

Closes: #40542

BUILDING.md

@@ -523,8 +523,8 @@ installing `ccache` can help to greatly reduce build times. Set up with:
 $ sudo apt install ccache   # for Debian/Ubuntu, included in most Linux distros
 $ ccache -o cache_dir=<tmp_dir>
 $ ccache -o max_size=5.0G
-$ export CC="ccache gcc"    # add to your .profile
-$ export CXX="ccache g++"   # add to your .profile
+$ export CC="ccache gcc"    # add to your .profile, use cc (not gcc) on macOS
+$ export CXX="ccache g++"   # add to your .profile, use c++ (not g++) on macOS
 ```
 
 This will allow for near-instantaneous rebuilds even when switching branches.