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

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

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

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[gdb/build] std::thread     |[gdb/build] std::thread
                   |support configure check     |support configure check
                   |does not used CXX_DIALECT   |does not use CXX_DIALECT
                 CC|                            |palves at sourceware dot org

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
We have a custom implementation of AX_CXX_COMPILE_STDCXX at
gdb/ax_cxx_compile_stdcxx.m4:
...
    Use AX_CXX_COMPILE_STDCXX to detect if the compiler supports C++11,
    and if -std=xxx switches are necessary to enable C++11.

    We need to tweak AX_CXX_COMPILE_STDCXX a bit though.  Pristine
    upstream AX_CXX_COMPILE_STDCXX appends -std=gnu++11 to CXX directly.
    That doesn't work for us, because 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.

    So instead of having AX_CXX_COMPILE_STDCXX set CXX directly, tweak it
    to AC_SUBST a separate variable -- CXX_DIALECT -- and use '$(CXX)
    (CXX_DIALECT)' to compile/link.
...
which introduced the CXX_DIALECT.

If we would use the regular AX_CXX_COMPILE_STDCXX, AFAIU the problem would not
occur.

So, either:
- we add CXX_DIALECT to all uses of CXX in the configure script, or 
- we add CXX_DIALECT to CXX directly.  AFAIU, that wouldn't break anything
  though it would result in confusing double options in command lines.

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