stella: fix compiler search in configure script

stella: fix compiler search in configure script

In configure script there are some tests that are looking for
a suitable compiler. For this purpose script tries to compile
and link some C++ code that is hardcoded in the body of script.

The problem is that only linker flags ($LDFLAGS) are passed to the
compiler when the script is compiling test code. Therefore some
necessary command line options can be lost and this would lead to
errors.

This exactly happens to ARC:
  1. -matomic option is lost as it compiler option,
  2. test code compilation fails,
  3. the script decides not to use arc-buildroot-linux-uclibc-g++,
  4. scipt checks host compiler (x86_64-linux-gnu-g++ in your case),
  5. package is being built for x86_64, not for ARC.

About last 2 steps - I don't know why configure script should try
to build stella for host machine. As for me an error should be
raised and a build should be stopped. But this is up to stella
developers.

Current patch adds compiler options ($CXXFLAGS) when compiling
test code in configure script.

Fixes stella build for ARC, e. g.:
http://autobuild.buildroot.net/results/c23/c23d655137b1c1825b1da69b18307c6a1d4b23b8//

[Peter: s/CPPFLAGS/CXXFLAGS/]
Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>