Merge bitcoin-core/secp256k1#1543: cmake: Do not modify build types when integrating by downstream project

158f9e5eae cmake: Do not modify build types when integrating by downstream project (Hennadii Stepanov)

Pull request description:

  The `CMAKE_BUILD_TYPE` and `CMAKE_CONFIGURATION_TYPES` must be managed by the downstream project.

  Suggesting to review with `git diff -w`.

  Fixes `std::out_of_range` exception from CMake in https://github.com/hebasto/bitcoin/pull/192 when running configuration step using "Ninja Multi-Config" generator:
  ```
  $ cmake -B build -G "Ninja Multi-Config"
  ...
  -- Configuring done (17.1s)
  terminate called after throwing an instance of 'std::out_of_range'
    what():  map::at
  Aborted (core dumped)
  ```

  Here are related discussions:
  - https://discourse.cmake.org/t/uncaught-exception-when-trying-to-generate-a-project-using-ninja-multi-config/11051
  - https://gitlab.kitware.com/cmake/cmake/-/issues/26064

ACKs for top commit:
  real-or-random:
    ACK 158f9e5eae

Tree-SHA512: b3040f40438d530f14b7e0f7d523e74b5843d88d250ff7955a99cc8c451feb9471a48134d1a89b3651b3f8195f91c17135c7b8a5d3ab092c8d35275b57743b8c

CMakeLists.txt

@@ -213,6 +213,7 @@ mark_as_advanced(
   CMAKE_SHARED_LINKER_FLAGS_COVERAGE
 )
 
+if(PROJECT_IS_TOP_LEVEL)
   get_property(is_multi_config GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
   set(default_build_type "RelWithDebInfo")
   if(is_multi_config)
@@ -232,6 +233,7 @@ else()
       )
     endif()
   endif()
+endif()
 
 include(TryAppendCFlags)
 if(MSVC)