configure: fix getentropy() for sierra and ios

This diff changes the logic by which configure detects getentropy() to
ensure that we don't use the system wide getentropy

- with macOS sierra if the deployment target is lower than sierra as
  found by tor developers here

    https://gitweb.torproject.org/tor.git/commit/?id=https://gitweb.torproject.org/tor.git/commit/?id=16fcbd21c963a9a65bf55024680c8323c8b7175d

- with iOS unconditionally because an app linking libressl compiled with
  system wide getentropy has been rejected by the App store as I have
  documented here

    https://github.com/measurement-kit/measurement-kit/pull/994

I think something similar could also affect clock_gettime judging from
tor's patch, but this diff for now doesn't address that.

I do not have macOS < sierra, so I could only verify that configure was
not picking up system wide getentropy by compiling libressl using

    export CFLAGS="-mmacosx-version-min=10.11"

As regards iOS, removing the check for getentropy and recompiling (thus
using libressl builtin getentropy()) was enough to have another iteration
of the app accepted. Otherwise testing should be possible with:

    export LDFLAGS=-arch armv7 -miphoneos-version-min=7.1 -isysroot `xcrun --show-sdk-path --sdk iphoneos`
    export CPPFLAGS=-arch armv7 -isysroot `xcrun --show-sdk-path --sdk iphoneos`
    export CFLAGS=-arch armv7 -miphoneos-version-min=7.1 -isysroot `xcrun --show-sdk-path --sdk iphoneos`

Related ticket: https://github.com/libressl-portable/portable/issues/230

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