AC_PROG_CXX should have a way to not fall back to g++

[Wouter Verhelst]

I received a bug report [1] that configure finished successfully, but
that the C++ compiler was not found. I noticed that this is documented
behaviour [2]: if AC_PROG_CXX doesn't find a C++ compiler, it falls back
to g++ as a default.

I'm not sure why this behaviour was chosen; however, I think it's a bad
idea. If configure runs to completion, people rightly expect that things
will compile successfully. On modern GNU/Linux distributions, users may
optionally not install the full GNU compiler collection; in such cases,
having a configure script that fails to notice that there's no C++
compiler is annoying.

I'm sure that for backwards compatibility, changing the default
behaviour of the AC_PROG_CXX macro is not an option right now. However,
it should be possible to make it accept a second parameter, or to
provide an alternative version of this macro (say, AC_PROG_CXX_REQUIRED
or some such) that allows to make it not exhibit this problematic
behaviour.

Thanks,

[1] https://github.com/Fedict/eid-mw/issues/78
[2] 
https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/C_002b_002b-Compiler.html#C_002b_002b-Compiler

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