Try to work around breakage in the OSX 10.12 SDK.

Apple is supposed to decorate their function declarations with
macros to indicate which OSX version introduced them, so that you
can tell the compiler that you want to build against certain
versions of OSX.  But they forgot to do that for clock_gettime() and
getentropy(), both of which they introduced in 10.12.

This patch adds a kludge to the configure.ac script where, if we
detect that we are targeting OSX 10.11 or earlier, we don't even probe
to see if the two offending functions are present.

Closes ticket 20235.