cc/gcc: set CXXFLAGS at configure gcc

Since gcc 4.8 C++ is also used as implementation language (see gcc
release notes).

Signed-off-by: "Daniel Dittmann" <ddittmann@gmx.net>
Message-Id: <acc7d11bc77b30f21c5b.1388863298@bernalk.machteam>
Patchwork-Id: 306883

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =