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.