configure: Handle having no c++ compiler in FORTIFY_SOURCE check

Our FORTIFY_SOURCE check assumes that $cxx refers to a working C++
compiler, with the result that if you don't happen to have one
then configure will spuriously print
  configure: line 4685: c++: command not found

Fix this by adding a 'has $cxx' check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
