configure: check that C++ compiler actually works

Check that the C++ compiler works with the C compiler; if it
does not, then don't pass CXX to the build process. This
fixes a regression where QEMU was no longer building if the
build environment didn't have a C++ compiler (introduced
in commit 3144f78b, which incorrectly assumed that rules.mak
would only see a non-empty $(CXX) if configure had actually
found a working C++ compiler).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reported-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Tested-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-id: 1392909016-14028-1-git-send-email-peter.maydell@linaro.org
