Dealing with compilers that pretend to be GCC

[Ludovic Courtès]

Hello,

A number of compilers claim to be GCC, without actually being GCC.  This
has come to a point where they can hardly be distinguished–until one
actually tries to use them.

I had the following macro to determine whether plug-in support is
available:

  https://gforge.inria.fr/scm/viewvc.php/trunk/m4/gcc.m4?view=markup&revision=5169&root=starpu&pathrev=5202

The macro is fairly elaborate.  Yet, ICC 12.0.1 and Clang 3.4 both pass
the test, because:

  - They support ‘--version’;

  - They define ‘__GNUC__’, which defeats Autoconf’s
    ‘_AC_LANG_COMPILER_GNU’.

  - They support ‘-print-file-name’, and have ‘-print-file-name=plugin’
    return GCC’s (!) plug-in header directory.  To that end, ICC simply
    runs ‘gcc -print-file-name=plugin’, while Clang appears to be doing
    some guesswork.

It turns out that ICC manages to build a working GCC plug-in, so after
all, it may be “entitled” to define ‘__GNUC__’, in a broad sense.

Conversely, Clang doesn’t support several GNU extensions, such as nested
functions, so it quickly fails to compile code.

Based on that, I modified my feature test like this:

  https://gforge.inria.fr/scm/viewvc.php/trunk/m4/gcc.m4?root=starpu&r1=5169&r2=5203

I don’t see what can be done on “our” side (perhaps Autoconf’s feature
test could be strengthened, but how?), but I wanted to document this
state of affairs.

Thanks,
Ludo’.