• Yann E. MORIN's avatar
    core/infra: fix build on toolchain without C++ · 434a3e49
    Yann E. MORIN authored
    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
        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's avatarDamien Riegel <damien.riegel@savoirfairelinux.com>
    Signed-off-by: default avatar"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>
    Signed-off-by: default avatarThomas Petazzoni <thomas.petazzoni@free-electrons.com>
    (cherry picked from commit bd39d11d2eaa679f09ab49fd3e4cd5511a168d1c)
    Signed-off-by: default avatarPeter Korsgaard <peter@korsgaard.com>