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

Wed Sep 19 20:55:54 UTC 2018

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

/Magnus