(NOTE: Not the macros, but the apparent decision of early Swift developers to divide the world into two camps: OS X and Linux. Moreover, glibc is conflated with Linux. Worse, glibc is the de facto portability layer. I want to cry....)
Well ... Haiku isn't exactly glibc. Our math and IO portions are indeed glibc at least in large part (although a very old version...) but there is also a large chunk (locale, wchar_t, etc.) which we have our own version of.