modules: print the optional modules as part of the meson summary

This makes it easier to figure out if a particular module was missing
from the build. Example output:

  Optional Modules
    rt                        : YES
    rtkit                     : YES
    portal                    : YES
    pulse-tunnel              : YES
    zeroconf-discover         : YES
    roc-sink                  : NO
    roc-source                : NO