Allow building the python module with system cares

When building the python module and using the new
GRPC_PYTHON_BUILD_SYSTEM_CARES env variable, the third party cares code
is not compiled. Instead, the cares 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

@@ -96,6 +96,12 @@ BUILD_WITH_SYSTEM_OPENSSL = os.environ.get('GRPC_PYTHON_BUILD_SYSTEM_OPENSSL',
 BUILD_WITH_SYSTEM_ZLIB = os.environ.get('GRPC_PYTHON_BUILD_SYSTEM_ZLIB',
                                         False)
 
+# Export this variable to use the system installation of cares. You need to
+# have the header files installed (in /usr/include/) and during
+# runtime, the shared libary must be installed
+BUILD_WITH_SYSTEM_CARES = os.environ.get('GRPC_PYTHON_BUILD_SYSTEM_CARES',
+                                         False)
+
 # Environment variable to determine whether or not to enable coverage analysis
 # in Cython modules.
 ENABLE_CYTHON_TRACING = os.environ.get(
@@ -169,6 +175,10 @@ 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'),)
 
+if BUILD_WITH_SYSTEM_CARES:
+  CORE_C_FILES = filter(lambda x: 'third_party/cares' not in x, CORE_C_FILES)
+  CARES_INCLUDE = (os.path.join('/usr', 'include'),)
+
 EXTENSION_INCLUDE_DIRECTORIES = (
     (PYTHON_STEM,) + CORE_INCLUDE + SSL_INCLUDE + ZLIB_INCLUDE +
     CARES_INCLUDE + ADDRESS_SORTING_INCLUDE)
@@ -184,6 +194,8 @@ if BUILD_WITH_SYSTEM_OPENSSL:
   EXTENSION_LIBRARIES += ('ssl', 'crypto',)
 if BUILD_WITH_SYSTEM_ZLIB:
   EXTENSION_LIBRARIES += ('z',)
+if BUILD_WITH_SYSTEM_CARES:
+  EXTENSION_LIBRARIES += ('cares',)
 
 DEFINE_MACROS = (
     ('OPENSSL_NO_ASM', 1), ('_WIN32_WINNT', 0x600),