AC_PROG_CXX doesn't actually check whether what it finds is a real live

AC_PROG_CXX doesn't actually check whether what it finds is a real live
C++ compiler (it might not be one on, for example, OS X, due to "cc"
being a C compiler, "CC" referring to "cc" due to the case-insensitivity
of the default OS X file system, and "CC" being one of the names checked
for in AC_PROG_CXX), so if we really need a C++ compiler, test it with a
program that a C compiler won't compile.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@50204 f5534014-38df-0310-8fa8-9805f1628bb7