Add -Wno-error=deprecated-register to gdb build flags

Add -Wno-error=deprecated-register to gdb build flags

As shown in PR 23022, building with clang-6 and Python 2 trips on the
fact that the Python 2 headers use the "register" keyword:

/usr/include/python2.7/unicodeobject.h:534:5: error: 'register' storage class specifier is deprecated and incompatible with C++17 [-Werror,-Wdeprecated-register]
    register PyObject *obj,     /* Object */
    ^~~~~~~~~

This patch adds -Wno-error=deprecated-register to our flags, so that we can
still see this class of warnings, but they don't cause a build failure.

gdb/ChangeLog:

	PR gdb/23022
	* warning.m4: Add -Wno-error=deprecated-register.
	* configure: Re-generate.