configure: bump required bitlbee version to 3.2.1

BitlBee 3.2.1 introduces two features/fixes:
  1. Installing of json library headers
  2. The HTTP client is now compliant with HTTP-1.1

With bitlbee-3.2.1, BITLBEE_VERSION_CODE was never set correctly, and
still reflects bitlbee-3.2. As a result, some more hackery was added
to correctly check for bitlbee-3.2.1.