[Bug math/22296] glibc 2.26: signbit build issue with Gcc 5.5.0 on x86_64

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https://sourceware.org/bugzilla/show_bug.cgi?id=22296

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=386e1c26ac473d6863133ab9cbe3bbda16c15816

commit 386e1c26ac473d6863133ab9cbe3bbda16c15816
Author: Romain Naour <romain.naour@gmail.com>
Date:   Mon Oct 16 23:21:56 2017 +0200

    Let signbit use the builtin in C++ mode with gcc < 6.x (bug 22296)

    When using gcc < 6.x, signbit does not use the type-generic
    __builtin_signbit builtin, instead it uses __MATH_TG.
    However, when library support for float128 is available, __MATH_TG uses
    __builtin_types_compatible_p, which is not available in C++ mode.

    On the other hand, libstdc++ undefines (in cmath) many macros from
    math.h, including signbit, so that it can provide its own functions.
    However, during its configure tests, libstdc++ just tests for the
    availability of the macros (it does not undefine them, nor does it
    provide its own functions).

    Finally, libstdc++ configure tests include math.h and get the definition
    of signbit that uses __MATH_TG (and __builtin_types_compatible_p).
    Since libstdc++ does not undefine the macros during its configure
    tests, they fail.

    This patch lets signbit use the builtin in C++ mode when gcc < 6.x is
    used. This allows the configure test in libstdc++ to work.

    Tested for x86_64.

        [BZ #22296]
        * math/math.h: Let signbit use the builtin in C++ mode with gcc
        < 6.x

    Cc: Gabriel F. T. Gomes <gftg@linux.vnet.ibm.com>
    Cc: Joseph Myers <joseph@codesourcery.com>

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog   |    6 ++++++
 math/math.h |    9 +++++++++
 2 files changed, 15 insertions(+), 0 deletions(-)

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