Merge bitcoin#31709: cmake: Fail if Libmultiprocess is missing when WITH_MULTIPROCESS=ON

c31166a cmake: Fail if `Libmultiprocess` is missing when `WITH_MULTIPROCESS=ON` (Hennadii Stepanov)

Pull request description:

  Fixes bitcoin#31708:
  ```
  $ cmake -B build -DWITH_MULTIPROCESS=ON
  -- The CXX compiler identification is GNU 13.3.0
  -- Detecting CXX compiler ABI info
  -- Detecting CXX compiler ABI info - done
  -- Check for working CXX compiler: /usr/bin/c++ - skipped
  -- Detecting CXX compile features
  -- Detecting CXX compile features - done
  -- Found SQLite3: /usr/include (found suitable version "3.45.1", minimum required is "3.7.17")
  CMake Error at CMakeLists.txt:146 (find_package):
    By not providing "FindLibmultiprocess.cmake" in CMAKE_MODULE_PATH this
    project has asked CMake to find a package configuration file provided by
    "Libmultiprocess", but CMake did not find one.

    Could not find a package configuration file provided by "Libmultiprocess"
    with any of the following names:

      LibmultiprocessConfig.cmake
      libmultiprocess-config.cmake

    Add the installation prefix of "Libmultiprocess" to CMAKE_PREFIX_PATH or
    set "Libmultiprocess_DIR" to a directory containing one of the above files.
    If "Libmultiprocess" provides a separate development package or SDK, be
    sure it has been installed.

  -- Configuring incomplete, errors occurred!
  ```

ACKs for top commit:
  vasild:
    ACK c31166a
  ryanofsky:
    Code review ACK c31166a
  TheCharlatan:
    ACK c31166a

Tree-SHA512: 503e6d7ff253c9ae95b13ff0649af7db97c74a97c04ca6fe88130defae251b94bfe9f4466300d3fab16397c7c8346b392a80a7b80a2d6517464a4eabe3aa40db

Author: fanquake

CMakeLists.txt

@@ -143,8 +143,8 @@ cmake_dependent_option(WITH_DBUS "Enable DBus support." ON "CMAKE_SYSTEM_NAME ST
 
 option(WITH_MULTIPROCESS "Build multiprocess bitcoin-node and bitcoin-gui executables in addition to monolithic bitcoind and bitcoin-qt executables. Requires libmultiprocess library. Experimental." OFF)
 if(WITH_MULTIPROCESS)
-  find_package(Libmultiprocess COMPONENTS Lib)
-  find_package(LibmultiprocessNative COMPONENTS Bin
+  find_package(Libmultiprocess REQUIRED COMPONENTS Lib)
+  find_package(LibmultiprocessNative REQUIRED COMPONENTS Bin
     NAMES Libmultiprocess
   )
 endif()