RFR: JDK-8210920 Native C++ tests are not using CXXFLAGS

[Tim Bell]

Magnus:

> Native C++ tests are not using CXXFLAGS. This is mostly apparent on
> solaris, where CC and cc are quite different. Fixing this made some new
> warnings appear on C++ tests for solstudio/gcc.
>
> Also, when fixing this, it became apparent that the
> *FLAGS_TESTLIB/TESTEXE is just clumsy. In the long term, these super
> combined flags should be broken up. In the short term, just use
> *FLAGS_JDK explicitly, since that's what we're doing anyway.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8210920
> WebRev:
> http://cr.openjdk.java.net/~ihse/JDK-8210920-fix-testlibs-CXX-flags/webrev.01

Looks good.

/Tim