[Buildroot] [PATCHv2] core: alternate solution to disable C++

[Peter Seiderer <ps.report@gmx.net>]

Hello Yann,

On Tue, 27 Mar 2018 13:00:22 +0200, "Yann E. MORIN" <yann.morin.1998@free.fr> wrote:

> Some packages that use libtool really need some love to be able to
> disable C++ support.
> 
> This is because libtool will want to call AC_PROG_CXXCPP as soon as CXX
> is set non-empty to something different from 'no'. Then, AC_PROG_CXXCPP
> will want a C++ preprocessor that works on valid input *and* fail on
> invalid input.
> 
> So, providing 'false' as the C++ compiler will then require that we do
> have a working C++ preprocessor. Which is totally counter-productive
> since we do not have a C++ compiler to start with...
> 
> bd39d11d2e (core/infra: fix build on toolchain without C++) was a
> previous attempt at fixing this, by using the host's C++ preprocessor.
> 
> However, that is very incorrect (that's my code, I can say so!) because
> the set of defines will most probably be different for the host and the
> target, thus causign all sorts of trouble. For example, on ARM we'd have
> to include different headers for soft-float vs hard-float, which is
> decided based on a macro, which is not defined for x86, and thus may
> redirect to the wrong (and missing) header.
> 
> Instead, we notice that libtool uses the magic value 'no' to decide that
> a C++ compiler is not available, in which case it skeips the call to
> AC_PROG_CXXCPP.
> 
> Given that 'no' is not provided by any package in Debian and
> derivatives, as well as in Fedora, we can assume that no system will
> have an executable called 'no'. Hence, we use that as a magic value to
> disable C++ detection altogether.
> 
> Fixes: #10846 (again)

Fixes the gnutls configure failure described in Bug-10846...

Tested-by: Peter Seiderer <ps.report@gmx.net>

Regards,
Peter

> 
> Reported-by: Damien Riegel <damien.riegel@savoirfairelinux.com>
> Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
> Cc: Damien Riegel <damien.riegel@savoirfairelinux.com>
> Cc: Peter Seiderer <ps.report@gmx.net>
> Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
> Cc: Peter Korsgaard <peter@korsgaard.com>
> Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
> 
> ---
> Changes v1 -> v2:
>   - add big fat comment...
> 
> ---
>  package/Makefile.in | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/package/Makefile.in b/package/Makefile.in
> index e387ce67fe..57fb47ea2e 100644
> --- a/package/Makefile.in
> +++ b/package/Makefile.in
> @@ -409,8 +409,16 @@ else
>  NLS_OPTS = --disable-nls
>  endif
>  
> +# We need anything that is invalid. Traditionally, we'd have used 'false' (and
> +# we did so in the past). However, that breaks libtool for packages that have
> +# optional C++ support (e.g. gnutls), because libtool will *require* a *valid*
> +# C++ preprocessor as long as CXX is not 'no'.
> +# Now, whether we use 'no' or 'false' for CXX as the same side effect: it is an
> +# invalid C++ compiler, and thus will cause detection of C++ to fail (which is
> +# expected and what we want), while at the same time taming libtool into
> +# silence.
>  ifneq ($(BR2_INSTALL_LIBSTDCPP),y)
> -TARGET_CONFIGURE_OPTS += CXX=false CXXCPP=cpp
> +TARGET_CONFIGURE_OPTS += CXX=no
>  endif
>  
>  ifeq ($(BR2_STATIC_LIBS),y)