hurd: make build-many-glibcs.py use mainline mig

Some warnings come from code generated by mig, so we need a very recent
version for now.

        * scripts/build-many-glibcs.py (checkout_vcs): Add mig repository
        URL, and run autoreconf, make it the default for now.

ChangeLog

@@ -19,6 +19,8 @@
 	timer_ptr2id to cast struct timer_node * to void *.
 	* scripts/build-many-glibcs.py [os == gnu] (build_gcc): Pass
 	--disable-libcilkrts to gcc configure.
+	(checkout_vcs): Add mig repository URL, and run autoreconf, make it the
+	default for now.
 	* sysdeps/mach/hurd/Makefile [$(subdir)==nis]: Add
 	-DUSE_BINDINGDIR=1 to CFLAGS-ypclnt.c.
 	* mach/Machrules (MIGFLAGS): Do not set -DMACH_IPC_COMPAT=0.

scripts/build-many-glibcs.py

@@ -704,7 +704,7 @@ class Context(object):
                             'linux': '4.14',
                             'mpc': '1.1.0',
                             'mpfr': '4.0.0',
-                            'mig': '1.8',
+                            'mig': 'vcs-mainline',
                             'gnumach': '1.8',
                             'hurd': 'vcs-mainline'}
         use_versions = {}
@@ -786,6 +786,13 @@ class Context(object):
             r = self.git_checkout(component, git_url, git_branch, update)
             self.fix_glibc_timestamps()
             return r
+        elif component == 'mig':
+            git_url = 'git://git.savannah.gnu.org/hurd/mig.git'
+            git_branch = 'master'
+            r = self.git_checkout(component, git_url, git_branch, update)
+            subprocess.run(['autoreconf', '-i'],
+                           cwd=self.component_srcdir(component), check=True)
+            return r
         elif component == 'hurd':
             git_url = 'git://git.savannah.gnu.org/hurd/hurd.git'
             git_branch = 'master'