Read Bazel's $XML_OUTPUT_FILE environment variable

If $XML_OUTPUT_FILE is set, and $GTEST_OUTPUT and --gtest_output are not
specified, produce output as if GTEST_OUTPUT=xml:$XML_OUTPUT_FILE had
been set.

googletest/include/gtest/internal/gtest-port.h

@@ -2547,9 +2547,9 @@ bool ParseInt32(const Message& src_text, const char* str, Int32* value);
 bool BoolFromGTestEnv(const char* flag, bool default_val);
 GTEST_API_ Int32 Int32FromGTestEnv(const char* flag, Int32 default_val);
 const char* StringFromGTestEnv(const char* flag, const char* default_val);
+std::string OutputFromGTestEnv(const char * default_val);
 
 }  // namespace internal
 }  // namespace testing
 
 #endif  // GTEST_INCLUDE_GTEST_INTERNAL_GTEST_PORT_H_
-