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.

changes/bug20235

@@ -0,0 +1,4 @@
+  o Minor features (compatibility):
+    - Work around a bug in the OSX 10.12 SDK that would prevent us
+      from successfully targetting earlier versions of OSX.
+      Resolves ticket 20235.

configure.ac

@@ -394,14 +394,12 @@ AC_CHECK_FUNCS(
         accept4 \
         backtrace \
         backtrace_symbols_fd \
-        clock_gettime \
 	eventfd \
 	explicit_bzero \
 	timingsafe_memcmp \
         flock \
         ftime \
         getaddrinfo \
-        getentropy \
         getifaddrs \
         getpass \
         getrlimit \
@@ -438,6 +436,34 @@ AC_CHECK_FUNCS(
 	_vscprintf
 )
 
+# Apple messed up when they added two functions functions in Sierra: they
+# forgot to decorate them with appropriate AVAILABLE_MAC_OS_VERSION
+# checks. So we should only probe for those functions if we are sure that we
+# are not targetting OSX 10.11 or earlier.
+AC_MSG_CHECKING([for a pre-Sierra OSX build target])
+AC_TRY_COMPILE([
+#ifdef __APPLE__
+#  include <AvailabilityMacros.h>
+#  ifndef MAC_OS_VERSION_10_12
+#    define MAC_OS_VERSION_10_12 101200
+#  endif
+#  if defined(MAC_OS_X_VERSION_MIN_REQUIRED)
+#    if MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_VERSION_10_12
+#      error "Running on Mac OSX 10.11 or earlier"
+#    endif
+#  endif
+#endif
+], [],
+   [on_macos_pre_10_12=no ; AC_MSG_RESULT([no])],
+   [on_macos_pre_10_12=yes; AC_MSG_RESULT([yes])])
+
+if test "$on_macos_pre_10_12" = "no"; then
+  AC_CHECK_FUNCS(
+        clock_gettime \
+        getentropy \
+  )
+fi
+
 if test "$bwin32" != "true"; then
   AC_CHECK_HEADERS(pthread.h)
   AC_CHECK_FUNCS(pthread_create)