CHECK_CXX_SOURCE_COMPILES ignores compiler flags on macOS

[tkemmer]

On macOS, the configuration script ignores additional compiler flags when performing C++ compilation checks, caused explicitly by:

ball/cmake/BALLConfiguration.cmake

Lines 104 to 107 in c66bb52

	IF(NOT APPLE)
	SET(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${BALL_PROJECT_COMPILE_FLAGS}")
	SET(CMAKE_REQUIRED_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS} ${BALL_PROJECT_COMPILE_DEFNS}")
	ENDIF()

This behavior results in some C++11 language features being disabled on macOS although being supported. Currently affected feature guards:

• BALL_HAS_THREAD_LOCAL
• BALL_HAS_NOEXCEPT
• BALL_HAS_STD_STRING_CONST_ITERATOR_INITLIST_INSERT

Removing the condition from the code above solves this problem. However, this causes the FindXDR CMake macro to fail, disabling persistence support as a consequence.