[Bug build/28318] [gdb/build] std::thread support configure check does not use CXX_DIALECT

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28318

--- Comment #5 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The gdb-11-branch branch has been updated by Tom de Vries
<vries@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=9b4d030ed52b52fe208edaa8be640f4dd359defc

commit 9b4d030ed52b52fe208edaa8be640f4dd359defc
Author: Tom de Vries <tdevries@suse.de>
Date:   Mon Oct 4 18:56:42 2021 +0200

    [gdb/build] Add CXX_DIALECT to CXX

    Say we use a gcc version that (while supporting c++11) does not support
c++11
    by default, and needs an -std setting to enable it.

    If gdb would use the default AX_CXX_COMPILE_STDCXX from autoconf-archive,
then
    we'd have:
    ...
    CXX="g++ -std=gnu++11"
    ...

    That mechanism however has the following problem (quoting from commit
    0bcda685399):
    ...
    the top level Makefile passes CXX down to subdirs, and that overrides
whatever
    gdb/Makefile may set CXX to.  The result would be that a make invocation
from
    the build/gdb/ directory would use "g++ -std=gnu++11" as expected, while a
    make invocation at the top level would not.
    ...

    Commit 0bcda685399 fixes this by using a custom AX_CXX_COMPILE_STDCXX which
    does:
    ...
    CXX=g++
    CXX_DIALECT=-std=gnu++11
    ...

    The problem reported in PR28318 is that using the custom instead of the
    default AX_CXX_COMPILE_STDCXX makes the configure test for std::thread
    support fail.

    We could simply add $CXX_DIALECT to the test for std::thread support, but
    that would have to be repeated for each added c++ support test.

    Instead, fix this by doing:
    ...
    CXX="g++ -std=gnu++11"
    CXX_DIALECT=-std=gnu++11
    ...

    This is somewhat awkward, since it results in -std=gnu++11 occuring twice
in
    some situations:
    ...
    $ touch src/gdb/dwarf2/read.c
    $ ( cd build/gdb; make V=1 dwarf2/read.o )
    g++-4.8 -std=gnu++11 -x c++ -std=gnu++11 ...
    ...

    However, both settings are needed:
     - the switch in CXX for the std::thread tests (and other tests)
     - the switch in CXX_DIALECT so it can be appended in Makefiles, to
       counteract the fact that the top-level Makefile overrides CXX

    The code added in gdb/ax_cxx_compile_stdcxx.m4 is copied from the default
    AX_CXX_COMPILE_STDCXX from autoconf-archive.

    Tested on x86_64-linux.

    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28318

    gdb/ChangeLog:

    2021-10-04  Tom de Vries  <tdevries@suse.de>

            PR build/28318
            * ax_cxx_compile_stdcxx.m4: Add CXX_DIALECT to CXX.
            * configure: Regenerate.

    gdbserver/ChangeLog:

    2021-10-04  Tom de Vries  <tdevries@suse.de>

            PR build/28318
            * configure: Regenerate.

    gdbsupport/ChangeLog:

    2021-10-04  Tom de Vries  <tdevries@suse.de>

            PR build/28318
            * configure: Regenerate.

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