runtime detection of getentropy for macOS via weak-linking (#3650)

* runtime detection of getentropy for macOS via weak-linking

In the before time, in the long long ago, there was a desire to use
getentropy on macOS. So some code was written and it detected getentropy
support by seeing if SYS_getentropy was available in the headers. But
lo, it turns out Apple ships headers for different SDK versions and
users on < 10.12 were getting headers that had SYS_getentropy even
though their OS did not support it. There was much wailing and
gnashing of teeth, but the frustrated developers remembered that Apple
wants their developers to use weak linking. With weak linking the mighty
developer can specify a minimum version and any symbol that was added
after that version will be weakly linked. Then, at runtime, the dynamic
linker will make unavailable symbols thus marked into NULLs. So, the
developer need only alter their code to do runtime detection of weakly
linked symbols and then a single binary may be compiled that will
correctly select getentropy or /dev/urandom at runtime. Hallelujah!

* oops

* separate the enum

* okay just apple

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