Merge #15584: build: disable BIP70 support by default

e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 doc: specify protobuf as optional in build docs (fanquake)
376f4929f8f75011b72b2f9c3164980db482278a build: disable BIP70 support by default (fanquake)

Pull request description:

  Disable BIP70 support in the GUI by default for `0.19.0` (for eventual removal in `0.20.0`?).

  Users who want to compile with BIP70 support enabled can pass `--enable-bip70` to `./configure`.

  I've inverted the current `--disable-bip70` test to instead pass `--enable-bip70`.

  Tested configurations on `macOS` (`protobuf` installed with `brew`).
  Protobuf available and `./configure`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = no
  ```

  Protobuf available and `./configure --enable-bip70`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = yes
  ```

  Protobuf not available (i.e `brew unlink protobuf`) and `./configure`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = no
  ```

  Protobuf not available and `./configure --enable-bip70`:
  ```
  checking whether to build test_bitcoin-qt... yes
  checking whether to build BIP70 support... configure: error: protobuf missing
  ```

  TODO:
  - [x] Remove `protobuf` from other Travis builds
  - [ ] Documentation updates (mention that `protobuf` is now optional)?
  - [ ] Could split release notes into GUI and build

ACKs for top commit:
  laanwj:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0
  elichai:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 Read the autotools changes. awesome that this removes the protobuf requirement.
  practicalswift:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 -- diff looks correct

Tree-SHA512: 7bf87ae8555e24db2da2e89cc4d4e90d09be27499ad386ad65879d05df8f96d9a1384379891ac8963d17728c90e55961560813df97e849e631e2de8c08e210c8

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