Fix gdbsupport build on compilers that don't default to C++11 or above

gdbsupport fails to build with compilers that don't default to C++11
or above.  gdbsupport's configure.ac is already using
AX_CXX_COMPILE_STDCXX, which sets CXX_DIALECT to the -std=gnu++11
switch if necessary, but the problem is that nowhere are we using
CXX_DIALECT.  This fixes it.

gdbsupport/ChangeLog:
2020-01-17   Pedro Alves  <palves@redhat.com>

	* Makefile.am: Append CXX_DIALECT to CXX.
	* Makefile.in: Regenerate.