scripts: Fix windows paths in helper.cmake

Normalize (all slashes going the same direction) and escape ("\" to "\\")
paths when writing helper.cmake.  CMake was previously interpreting the
single '\' characters and whatever followed as invalid escape sequences.
Linux and macOS are unaffected.
diff --git a/scripts/update_deps.py b/scripts/update_deps.py
index 119cf70..5d00eb5 100755
--- a/scripts/update_deps.py
+++ b/scripts/update_deps.py
@@ -306,9 +306,9 @@
         self.build_dir = None
         self.install_dir = None
         if json.get('build_dir'):
-            self.build_dir = json['build_dir']
+            self.build_dir = os.path.normpath(json['build_dir'])
         if json.get('install_dir'):
-            self.install_dir = json['install_dir']
+            self.install_dir = os.path.normpath(json['install_dir'])
         self.deps = json['deps'] if ('deps' in json) else []
         self.prebuild = json['prebuild'] if ('prebuild' in json) else []
         self.prebuild_linux = json['prebuild_linux'] if (
@@ -520,6 +520,8 @@
     This information is baked into the CMake files of the home repo and so
     this dictionary is kept with the repo via the json file.
     """
+    def escape(path):
+        return path.replace('\\', '\\\\')
     install_names = GetInstallNames(args)
     with open(filename, 'w') as helper_file:
         for repo in repos:
@@ -527,7 +529,7 @@
                 helper_file.write('set({var} "{dir}" CACHE STRING "" FORCE)\n'
                                   .format(
                                       var=install_names[repo.name],
-                                      dir=repo.install_dir))
+                                      dir=escape(repo.install_dir)))
 
 
 def main():