diff -r 000000000000 -r 6474c204b198 build/subconfigure.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/build/subconfigure.py	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,137 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# This script is used to capture the content of config.status-generated
+# files and subsequently restore their timestamp if they haven't changed.
+
+import os
+import re
+import subprocess
+import sys
+import pickle
+
+class File(object):
+    def __init__(self, path):
+        self._path = path
+        self._content = open(path, 'rb').read()
+        stat = os.stat(path)
+        self._times = (stat.st_atime, stat.st_mtime)
+
+    @property
+    def path(self):
+        return self._path
+
+    @property
+    def mtime(self):
+        return self._times[1]
+
+    def update_time(self):
+        '''If the file hasn't changed since the instance was created,
+           restore its old modification time.'''
+        if not os.path.exists(self._path):
+            return
+        if open(self._path, 'rb').read() == self._content:
+            os.utime(self._path, self._times)
+
+
+# As defined in the various sub-configures in the tree
+PRECIOUS_VARS = set([
+    'build_alias',
+    'host_alias',
+    'target_alias',
+    'CC',
+    'CFLAGS',
+    'LDFLAGS',
+    'LIBS',
+    'CPPFLAGS',
+    'CPP',
+    'CCC',
+    'CXXFLAGS',
+    'CXX',
+    'CCASFLAGS',
+    'CCAS',
+])
+
+
+# Autoconf, in some of the sub-configures used in the tree, likes to error
+# out when "precious" variables change in value. The solution it gives to
+# straighten things is to either run make distclean or remove config.cache.
+# There's no reason not to do the latter automatically instead of failing,
+# doing the cleanup (which, on buildbots means a full clobber), and
+# restarting from scratch.
+def maybe_clear_cache():
+    comment = re.compile(r'^\s+#')
+    cache = {}
+    with open('config.cache') as f:
+        for line in f.readlines():
+            if not comment.match(line) and '=' in line:
+                key, value = line.split('=', 1)
+                cache[key] = value
+    for precious in PRECIOUS_VARS:
+        entry = 'ac_cv_env_%s_value' % precious
+        if entry in cache and (not precious in os.environ or os.environ[precious] != cache[entry]):
+            os.remove('config.cache')
+            return
+
+
+def dump(dump_file, shell):
+    if os.path.exists('config.cache'):
+        maybe_clear_cache()
+    if not os.path.exists('config.status'):
+        if os.path.exists(dump_file):
+            os.remove(dump_file)
+        return
+
+    config_files = [File('config.status')]
+
+    # Scan the config.status output for information about configuration files
+    # it generates.
+    config_status_output = subprocess.check_output(
+        [shell, '-c', './config.status --help'],
+        stderr=subprocess.STDOUT).splitlines()
+    state = None
+    for line in config_status_output:
+        if line.startswith('Configuration') and line.endswith(':'):
+            state = 'config'
+        elif not line.startswith(' '):
+            state = None
+        elif state == 'config':
+            for f in (couple.split(':')[0] for couple in line.split()):
+                if os.path.isfile(f):
+                    config_files.append(File(f))
+
+    with open(dump_file, 'wb') as f:
+        pickle.dump(config_files, f)
+
+
+def adjust(dump_file, configure):
+    if not os.path.exists(dump_file):
+        return
+
+    config_files = []
+
+    try:
+        with open(dump_file, 'rb') as f:
+            config_files = pickle.load(f)
+    except Exception:
+        pass
+
+    for f in config_files:
+        # Still touch config.status if configure is newer than its original
+        # mtime.
+        if configure and os.path.basename(f.path) == 'config.status' and \
+                os.path.getmtime(configure) > f.mtime:
+            continue
+        f.update_time()
+
+    os.remove(dump_file)
+
+
+CONFIG_DUMP = 'config_files.pkl'
+
+if __name__ == '__main__':
+    if sys.argv[1] == 'dump':
+        dump(CONFIG_DUMP, sys.argv[2])
+    elif sys.argv[1] == 'adjust':
+        adjust(CONFIG_DUMP, sys.argv[2] if len(sys.argv) > 2 else None)