Update setup-building.rst with a note about missing libmpdec-dev

Add a note to the linux/debian instructions for building all optional packages.  libmpdec-dev is removed from recent Debian and can safely be dropped from list of packages to install.

getting-started/setup-building.rst

@@ -660,6 +660,9 @@ on Linux, macOS and iOS.
             libncurses5-dev libreadline6-dev libsqlite3-dev libssl-dev \
             lzma lzma-dev tk-dev uuid-dev zlib1g-dev libmpdec-dev
 
+   Note that recent Debian distributions do not have the ``libmpdec-dev`` package.
+   You can safely remove it from the install list above and the Python build will
+   use a bundled version.
 
 .. tab:: macOS