CXXSTD is the C++ analogue to CSTD. It defaults to `-std=c++11` with supporting
compilers; `-std=c++89`gnu++98`, otherwise (for g++ 4.2.1)for older versions of g++.
This change standardizes the CXXSTD variable, added to googletest.test.inc.mk
as part of r345203.
MFC after: 1 month