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.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2018-04-06  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	PR gdb/23022
+	* warning.m4: Add -Wno-error=deprecated-register.
+	* configure: Re-generate.
+
 2018-04-05  Tom Tromey  <tom@tromey.com>
 
 	* linespec.h: Remove include of "vec.h".

gdb/configure

@@ -15358,7 +15358,8 @@ build_warnings="-Wall -Wpointer-arith \
 -Wno-switch -Wno-char-subscripts \
 -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable \
 -Wno-sign-compare -Wno-narrowing -Wno-error=maybe-uninitialized \
--Wno-mismatched-tags"
+-Wno-mismatched-tags \
+-Wno-error=deprecated-register"
 
 case "${host}" in
   *-*-mingw32*)

gdb/warning.m4

@@ -41,7 +41,8 @@ build_warnings="-Wall -Wpointer-arith \
 -Wno-switch -Wno-char-subscripts \
 -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable \
 -Wno-sign-compare -Wno-narrowing -Wno-error=maybe-uninitialized \
--Wno-mismatched-tags"
+-Wno-mismatched-tags \
+-Wno-error=deprecated-register"
 
 case "${host}" in
   *-*-mingw32*)