bpo-35813: Added shared_memory submodule of multiprocessing. (#11664)

Added shared_memory submodule to multiprocessing in time for first alpha with cross-platform tests soon to follow.

Misc/NEWS.d/next/Library/2019-01-23-22-44-37.bpo-35813.Yobj-Y.rst

+Shared memory submodule added to multiprocessing to avoid need for
+serialization between processes

setup.py

@@ -1592,6 +1592,17 @@ class PyBuildExt(build_ext):
             if (sysconfig.get_config_var('HAVE_SEM_OPEN') and not
                 sysconfig.get_config_var('POSIX_SEMAPHORES_NOT_ENABLED')):
                 multiprocessing_srcs.append('_multiprocessing/semaphore.c')
+            if (self.compiler.find_library_file(lib_dirs, 'rt') or
+                host_platform != 'cygwin'):
+                posixshmem_srcs = [ '_multiprocessing/posixshmem.c',
+                                  ]
+                libs = []
+                if self.compiler.find_library_file(lib_dirs, 'rt'):
+                    libs.append('rt')
+                exts.append( Extension('_posixshmem', posixshmem_srcs,
+                                       define_macros={},
+                                       libraries=libs,
+                                       include_dirs=["Modules/_multiprocessing"]))
 
         exts.append ( Extension('_multiprocessing', multiprocessing_srcs,
                                 define_macros=list(macros.items()),