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.)
