Add ESP8266 configs to PlatformIO build

Added various conditional compliations for ESP8266 to stub out
missing functionality.

googlemock/src/gmock_main.cc

@@ -32,7 +32,10 @@
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
-#if defined GTEST_OS_ESP8266 || defined GTEST_OS_ESP32
+#if GTEST_OS_ESP8266 || GTEST_OS_ESP32
+# if GTEST_OS_ESP8266
+extern "C" {
+# endif
 void setup() {
   // Since Google Mock depends on Google Test, InitGoogleMock() is
   // also responsible for initializing Google Test.  Therefore there's
@@ -40,6 +43,10 @@ void setup() {
   testing::InitGoogleMock();
 }
 void loop() { RUN_ALL_TESTS(); }
+# if GTEST_OS_ESP8266
+}
+# endif
+
 #else
 
 // MS C++ compiler/linker has a bug on Windows (not on Windows CE), which