Autoconf 2.69 AC_PROG_CXX accepts plain C compilers

[Peter De Wachter]

I was testing a C++ project of mine with older versions of GCC, and
mistakingly used the following command:
$ ./configure CXX=gcc-4.7

Autoconf didn't spot any problem:
> checking whether the C++ compiler works... yes
But of course gcc-4.7 doesn't know how to link a C++ program, so my
build failed.

The attached patch fixes this problem by changing the C++ null program
into:

int main() { (void) new int; return 0; }

Any C++ compiler, even an ancient one, should be able to pass that test,
but C compilers will fail. In GCC's case, the program will compile but
the link will fail because libc doesn't contain an 'operator new'.

null_cxx_program.patch
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