$GXX = yes when using non-GCC compiler

[Peter Johansson]

Hello autoconfers,

In the section where manual describes AC_PROG_CXX

http://www.gnu.org/s/hello/manual/autoconf/C_002b_002b-Compiler.html#C_002b_002b-Compiler

it claims that shell variable GXX is set to ‘yes’ if using GNU C++ compiler.

This seems to be inaccurate because when I build with, e.g., Intel compiler
I get GXX set to yes as well. Looking into the code  of macro
_AC_LANG_COMPILER_GNU that sets this variable it looks like it relies on
whether PP macro __GNUC__ is defined or not. It turns out that this is not
a good way to determine whether GCC is used because many compilers defined
that macro:

http://gcc.gnu.org/ml/gcc/2008-07/threads.html#00025

Obviously it would be good if CXX behaves as documented but before that can
be fixed I think the manual perhaps should be updated.

Thank you,

Peter