cmake: prevent overriding CMAKE_BUILD_TYPE if it is subdirectory

[recp]

We have this configuration:

set(DEFAULT_BUILD_TYPE "Release")

...

if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
  message(STATUS "Setting build type to '${DEFAULT_BUILD_TYPE}' as none was specified.")
  set(CMAKE_BUILD_TYPE "${DEFAULT_BUILD_TYPE}" CACHE STRING "Choose the type of build." FORCE)
  # Set the possible values of build type for cmake-gui
  set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS "Debug" "Release" "MinSizeRel" "RelWithDebInfo")
endif()

this overrides parent project if parent project don't set BUILD_TYPE e.g Debug. The PR adds additional checks before set build type:

get_directory_property(hasParent PARENT_DIRECTORY)

if(NOT hasParent AND NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
...

fixes #275

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references:

• https://stackoverflow.com/questions/25199677/how-to-detect-if-current-scope-has-a-parent-in-cmake