--- a/addon-sdk/Makefile.in
+++ b/addon-sdk/Makefile.in
@@ -4,16 +4,18 @@
 
 DEPTH     = @DEPTH@
 topsrcdir = @top_srcdir@
 srcdir    = @srcdir@
 VPATH     = @srcdir@
 
 include $(topsrcdir)/config/config.mk
 
+export PYMAKE = $(.PYMAKE)
+
 libs::
 	$(PYTHON) $(srcdir)/copy_source.py $(topsrcdir) $(srcdir)/source/lib $(FINAL_TARGET)/modules/commonjs >copy_source.mk
 	$(MAKE) -f copy_source.mk libs
 
 include $(topsrcdir)/config/rules.mk
 
 TEST_FILES = \
   source/app-extension \

--- a/addon-sdk/copy_source.py
+++ b/addon-sdk/copy_source.py
@@ -1,21 +1,38 @@
 # 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/.
 
 import os
+import posixpath
 import sys
 
+
+def normpath(path):
+    """Ensure UNIX style paths are used with GNU make on Windows.
+
+    This can be removed once we no longer support GNU make on Windows (bug
+    828317).
+    """
+    if os.environ.get('PYMAKE') or os.name not in ('nt', 'ce'):
+        return path
+
+    if len(path) > 2 and path[1] == ':':
+        path = '/' + path[0] + path[2:]
+
+    return posixpath.normpath(path)
+
+
 if len(sys.argv) != 4:
     print >> sys.stderr, "Usage: copy_source.py " \
                          "<topsrcdir> <source directory> <target directory>"
     sys.exit(1)
 
-topsrcdir = sys.argv[1]
+topsrcdir = normpath(sys.argv[1])
 source_dir = sys.argv[2]
 target_dir = sys.argv[3]
 
 print """
 DEPTH     = ..
 topsrcdir = %(topsrcdir)s
 srcdir    = %(topsrcdir)s/addon-sdk
 VPATH     = %(topsrcdir)s/addon-sdk