gdb: Require C++11

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.

Confirmed that this enables C++11 starting with gcc 4.8, the first gcc
release with full C++11 support.

Also confirmed that configure errors out gracefully with older GCC
releases:

  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features by default... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -std=gnu++11... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -std=gnu++0x... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -std=c++11... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -std=c++0x... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with +std=c++11... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -h std=c++11... no
  configure: error: *** A compiler with support for C++11 language features is required.
  Makefile:9451: recipe for target 'configure-gdb' failed
  make[1]: *** [configure-gdb] Error 1
  make[1]: Leaving directory '/home/pedro/brno/pedro/gdb/mygit/cxx-convertion/build-gcc-4.7'

If we need to revert back to making C++11 optional, all that's
necessary is to change the "mandatory" to "optional" in configure.ac
and regenerate configure (both gdb and gdbserver).

gdb/ChangeLog:
2016-10-28  Pedro Alves  <palves@redhat.com>

	* Makefile.in (CXX_DIALECT): Get from configure.
	(COMPILE.pre, CC_LD): Append $(CXX_DIALECT).
	(FLAGS_TO_PASS): Pass CXX_DIALECT.
	* acinclude.m4: Include ax_cxx_compile_stdcxx.m4.
	* ax_cxx_compile_stdcxx.m4: Add FSF copyright header.  Set and
	AC_SUBST CXX_DIALECT instead of changing CXX/CXXCPP.
	* configure.ac: Call AX_CXX_COMPILE_STDCXX.
	* config.in: Regenerate.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2016-10-28  Pedro Alves  <palves@redhat.com>

	* Makefile.in (CXX_DIALECT): Get from configure.
	(COMPILE.pre, CC_LD): Append $(CXX_DIALECT).
	* acinclude.m4: Include ../ax_cxx_compile_stdcxx.m4.
	* configure.ac: Call AX_CXX_COMPILE_STDCXX.
	* config.in: Regenerate.
	* configure: Regenerate.

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