address comments by ericgribkoff@

o re-add USE_PREBUILT_GRPC_CORE option that was erroneously removed
o fix typo in comment

setup.py

@@ -265,6 +265,13 @@ def cython_extensions_and_necessity():
                   for name in CYTHON_EXTENSION_MODULE_NAMES]
   config = os.environ.get('CONFIG', 'opt')
   prefix = 'libs/' + config + '/'
+  if USE_PREBUILT_GRPC_CORE:
+    extra_objects = [prefix + 'libares.a',
+                     prefix + 'libboringssl.a',
+                     prefix + 'libgpr.a',
+                     prefix + 'libgrpc.a']
+    core_c_files = []
+  else:
     core_c_files = list(CORE_C_FILES)
     extra_objects = []
   extensions = [

src/python/grpcio/commands.py

@@ -215,7 +215,7 @@ class BuildExt(build_ext.build_ext):
 
         def compiler_ok_with_extra_std():
             """Test if default compiler is okay with specifying c++ version
-            when invokec in C mode. GCC is okay with this, while clang is not.
+            when invoked in C mode. GCC is okay with this, while clang is not.
             """
             cc_test = subprocess.Popen(
                 ['cc', '-x', 'c', '-std=c++11', '-'],