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

reporter_name="Yann Diorcet <diorcet.yann@gmail.com>"

reporter_url=""

reporter_comment="Config to build cross-compiler with mingw as kernel type for

64 bits Windows target"