Use proper feature test macro to test if library supports char8_t

Reference is here: https://en.cppreference.com/w/cpp/feature_test

This PR fixes the weird case of compiling with `clang++ -std=c++17 -fchar8_t`

Author: georgthegreat

googletest/include/gtest/gtest-printers.h

@@ -349,7 +349,7 @@ GTEST_IMPL_FORMAT_C_STRING_AS_POINTER_(char);
 GTEST_IMPL_FORMAT_C_STRING_AS_POINTER_(const char);
 GTEST_IMPL_FORMAT_C_STRING_AS_POINTER_(wchar_t);
 GTEST_IMPL_FORMAT_C_STRING_AS_POINTER_(const wchar_t);
-#ifdef __cpp_char8_t
+#ifdef __cpp_lib_char8_t
 GTEST_IMPL_FORMAT_C_STRING_AS_POINTER_(char8_t);
 GTEST_IMPL_FORMAT_C_STRING_AS_POINTER_(const char8_t);
 #endif