Allow building the python module with system zlib

When building the python module and using the new
GRPC_PYTHON_BUILD_SYSTEM_ZLIB env variable, the third party zlib code
is not compiled. Instead, the zlib shared library installed on the
system is used during runtime.
This is useful for distributions who don't want to include code copies
but use shared libraries instead.

setup.py

@@ -90,6 +90,12 @@ BUILD_WITH_CYTHON = os.environ.get('GRPC_PYTHON_BUILD_WITH_CYTHON', False)
 BUILD_WITH_SYSTEM_OPENSSL = os.environ.get('GRPC_PYTHON_BUILD_SYSTEM_OPENSSL',
                                            False)
 
+# Export this variable to use the system installation of zlib. You need to
+# have the header files installed (in /usr/include/) and during
+# runtime, the shared libary must be installed
+BUILD_WITH_SYSTEM_ZLIB = os.environ.get('GRPC_PYTHON_BUILD_SYSTEM_ZLIB',
+                                        False)
+
 # Environment variable to determine whether or not to enable coverage analysis
 # in Cython modules.
 ENABLE_CYTHON_TRACING = os.environ.get(
@@ -159,6 +165,10 @@ if BUILD_WITH_SYSTEM_OPENSSL:
   CORE_C_FILES = filter(lambda x: 'src/boringssl' not in x, CORE_C_FILES)
   SSL_INCLUDE = (os.path.join('/usr', 'include', 'openssl'),)
 
+if BUILD_WITH_SYSTEM_ZLIB:
+  CORE_C_FILES = filter(lambda x: 'third_party/zlib' not in x, CORE_C_FILES)
+  ZLIB_INCLUDE = (os.path.join('/usr', 'include'),)
+
 EXTENSION_INCLUDE_DIRECTORIES = (
     (PYTHON_STEM,) + CORE_INCLUDE + SSL_INCLUDE + ZLIB_INCLUDE +
     CARES_INCLUDE + ADDRESS_SORTING_INCLUDE)
@@ -172,6 +182,8 @@ if "win32" in sys.platform:
   EXTENSION_LIBRARIES += ('advapi32', 'ws2_32',)
 if BUILD_WITH_SYSTEM_OPENSSL:
   EXTENSION_LIBRARIES += ('ssl', 'crypto',)
+if BUILD_WITH_SYSTEM_ZLIB:
+  EXTENSION_LIBRARIES += ('z',)
 
 DEFINE_MACROS = (
     ('OPENSSL_NO_ASM', 1), ('_WIN32_WINNT', 0x600),