Add --with-custom-branding and --with-custom-version options

Based on the following commit (didn't apply cleanly, so changes changes
were applied manually):

  atheme/charybdis/b6d0720c1cbc7b19e68268fa3235c3a54485f971
    configure: Add --with-custom-branding and --with-custom-version
    options.
Merge remote-tracking branch 'origin/master'