AC_PROG_CXX doesn't check whether what it finds is actually a C++ compiler

[Guy Harris]

If you are on a system that

        1) doesn't have a program named g++, c++, gpp, or aCC installed;

        2) does have a C compiler named cc installed;

        3) has that program on a case-insensitive file system;

AC_PROG_CXX will misidentify cc as being a C++ compiler, as its search for CC 
will find cc (due to the case-insensitive file system) but will not try to 
compile a program containing C++-specific features with it.

(This was found by a somewhat artificial test, in which, on an OS X system with 
the default case-insensitive root file system, I temporarily renamed g++ and 
c++ so that AC_PROG_CXX wouldn't find them, and ran a configure script.  
Whether this is something to be concerned about in the real world is another 
matter.)