Use MSVC_FLAGS with MSVC. (#57)

GCC_FLAGS don't work (we get an error about /Wextra)

Author: g-easy

opencensus/BUILD

@@ -21,3 +21,11 @@ config_setting(
     },
     visibility = [":__subpackages__"],
 )
+
+config_setting(
+    name = "windows",
+    values = {
+        "cpu": "x64_windows",
+    },
+    visibility = [":__subpackages__"],
+)

opencensus/copts.bzl

@@ -27,14 +27,18 @@ load(
     "GCC_TEST_FLAGS",
     "LLVM_FLAGS",
     "LLVM_TEST_FLAGS",
+    "MSVC_FLAGS",
+    "MSVC_TEST_FLAGS",
 )
 
 DEFAULT_COPTS = select({
     "//opencensus:llvm_compiler": LLVM_FLAGS,
+    "//opencensus:windows": MSVC_FLAGS,
     "//conditions:default": GCC_FLAGS,
 })
 
 TEST_COPTS = DEFAULT_COPTS + select({
     "//opencensus:llvm_compiler": LLVM_TEST_FLAGS,
+    "//opencensus:windows": MSVC_TEST_FLAGS,
     "//conditions:default": GCC_TEST_FLAGS,
 })