cmake: Do not modify build types when integrating by downstream project

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

Suggesting to review with `git diff -w`.

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)