Skip Semaphore#value test on Mac OS X where it is unsupported.

spec/process_shared/semaphore_spec.rb

@@ -58,6 +58,7 @@ module ProcessShared
     end
 
     describe '#post and #wait' do
+      unless FFI::Platform.mac?
         it 'increments and decrements the value' do
           Semaphore.open(0) do |sem|
             10.times do |i|
@@ -72,6 +73,7 @@ module ProcessShared
           end
         end
       end
+    end
 
     describe '#try_wait' do
       it 'returns immediately with non-zero semaphore' do