[Buildroot] [PATCH RFC] core/infra: fix build on toolchain without C++

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Hello,

On Wed,  3 Jan 2018 18:39:52 +0100, Yann E. MORIN wrote:
> Autotools-based packages that do not need C++ but check for it, and use
> libtool, will fail to configure on distros that lack /lib/cpp.
> 
> This is the case for example on Arch Linux, where expat fails to build
> with:
> 
>     configure: error: in `/home/dkc/src/buildroot/build/build/expat-2.2.4':
>     configure: error: C++ preprocessor "/lib/cpp" fails sanity check
> 
> This is because libtool uses AC_PROC_CXXCPP, which can not be avoided,
> and does require a cpp that passes some "sanity" checks (does not choke
> on valid input, but does choke on invalid input). So we can use neither
> /bin/false nor /bin/true...
> 
> We instead need something that can digest some basic C++ preprocessor
> input. We can't use the target preprocessor: that does not work, because
> it obviously has no C++ cupport:
> 
>     arm-linux-cpp.br_real: error: conftest.cpp: C++ compiler not
>     installed on this system
> 
> We can however consider that the host machine does have a C++ compiler,
> so we use the host' cpp, which is gcc's compiler wrapper that ends up
> calling the host's C++ preprocessor.
> 
> That would give us a valid C++ preprocessor when we don't have one, in
> fact. But autotools will then correctly fail anyway, because there is
> indeed no C++ compiler at all, as we can see in this excerpt of a
> configure log from expat:
> 
>     checking whether we are using the GNU C++ compiler... no
>     checking whether false accepts -g... no
>     checking dependency style of false... none
>     checking how to run the C++ preprocessor... cpp
>     checking whether the false linker (/home/ymorin/dev/buildroot/O/host/bin/arm-linux-ld) supports shared libraries... yes
>     libtool.m4: error: problem compiling CXX test program
>     checking for false option to produce PIC...  -DPIC
>     checking if false PIC flag  -DPIC works... no
>     checking if false static flag  works... no
>     checking if false supports -c -o file.o... no
>     checking if false supports -c -o file.o... (cached) no
>     checking whether the false linker (/home/ymorin/dev/buildroot/O/host/bin/arm-linux-ld) supports shared libraries... yes
> 
> So, using the host's C++ preprocessor (by way of gcc's wrapper) leads to
> a working situation, where the end result is as expected.
> 
> 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: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
> Cc: Peter Korsgaard <peter@korsgaard.com>
> Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
> ---
>  package/Makefile.in | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Applied to master, thanks.

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com