Merge #128: cmake: Disable WITH_EXTERNAL_SIGNER when FUZZ=ON

d96f51a fixup! cmake: Add fuzzing options (Hennadii Stepanov)

Pull request description:

  The behavior is aligned with the master branch.

  First noticed in #89 (review).

ACKs for top commit:
  dergoegge:
    utACK d96f51a

Tree-SHA512: 9ba8ab9c4c14814f09c583e28c8bd3e0c8bc2d2ab83b79316b771ef376cb1dd4b8de8bbbd0777367d5e8510332471353da95242d74434d6b30bfdb8e285b2efc

CMakeLists.txt

@@ -132,6 +132,7 @@ if(FUZZ)
   set(BUILD_SHARED_LIBS OFF)
   set(BUILD_WALLET_TOOL OFF)
   set(WITH_GUI OFF)
+  set(WITH_EXTERNAL_SIGNER OFF)
   set(WITH_NATPMP OFF)
   set(WITH_MINIUPNPC OFF)
   set(WITH_ZMQ OFF)