Remove unnecessary _DARWIN_C_SOURCE definition in CMake files

[filipnavara]

We define this globally:

runtime/eng/native/configurecompiler.cmake

Lines 301 to 304 in d44116e

	# enable support for X/Open and POSIX APIs, like the <ucontext.h> header file
	add_definitions(-D_XOPEN_SOURCE)
	# enable support for Darwin extension APIs, like pthread_getthreadid_np
	add_definitions(-D_DARWIN_C_SOURCE)

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[filipnavara]

cc @akoeplinger This has been sitting here without any traction for weeks. Do you know who should review it? Thanks!

[filipnavara]

Thanks!