Fixed ./configure on Windows (tensorflow#4449)

* Fixed ./configure on Windows

Basically, symlink doesn't work on Windows

* Fixed shebang of gen_git_source.py

* gen_git_source.py: Replace os.popen with subprocess.check_output

os.popen doesn't work with Bazel properly on Windows, due to this error:
    RuntimeError: Cannot locate a COMSPEC environment variable to use as the shell

tensorflow/tools/git/gen_git_source.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright 2016 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -30,6 +30,7 @@
 import argparse
 import json
 import os
+import subprocess
 import shutil
 
 
@@ -111,7 +112,10 @@ def configure(src_base_path, debug=False):
     if src is None:
       open(os.path.join(gen_path, target), "w").write("")
     else:
-      os.symlink(src, os.path.join(gen_path, target))
+      if hasattr(os, 'symlink'):
+        os.symlink(src, os.path.join(gen_path, target))
+      else:
+        shutil.copy2(src, os.path.join(gen_path, target))
 
   json.dump(spec, open(os.path.join(gen_path, "spec.json"), "w"), indent=2)
   if debug:
@@ -157,9 +161,8 @@ def generate(arglist):
       raise RuntimeError(
           "Run ./configure again, branch was '%s' but is now '%s'" %
           (old_branch, new_branch))
-    strs["tf_git_version"] = os.popen(
-        "git -C \"%s\" describe --long --dirty --tags" %
-        (data["path"],)).read().strip()
+    strs["tf_git_version"] = subprocess.check_output(
+        ["git", "-C", data["path"], "describe", "--long", "--dirty", "--tags"]).strip()
   # TODO(aselle): Check for escaping
   cpp_file = "\n".join("const char* %s() {return \"%s\";}" % (x, y)
                        for x, y in strs.items())
@@ -177,7 +180,7 @@ def raw_generate(output_file):
   """
 
   strs = {"tf_compiler_version": "__VERSION__"}
-  version = os.popen("git describe --long --dirty --tags").read().strip()
+  version = subprocess.check_output(["git", "describe", "--long", "--dirty", "--tags"]).strip()
   version = version if version else "unknown"
   strs["tf_git_version"] = version
   cpp_file = "\n".join("const char* %s() {return \"%s\";}" % (x, y)

util/python/python_config.sh

@@ -142,10 +142,13 @@ function setup_python {
 
   for x in $EXPECTED_PATHS; do
     if [ -e "$x" ]; then
-      rm "$x"
+      # This makes ./configure slow on Windows, but it works.
+      rm -rf "$x"
     fi
   done
 
+# ln -sf is acutally implemented as copying in msys since creating symbolic links is privileged on Windows
+# So we need -rf to remove them above.
   ln -sf "${python_include}" util/python/python_include
   ln -sf "${python_lib}" util/python/python_lib
   ln -sf "${numpy_include}" third_party/py/numpy/numpy_include
@@ -159,13 +162,24 @@ function setup_python {
   echo "export PYTHON_BIN_PATH=$PYTHON_BIN_PATH" > tools/python_bin_path.sh
 }
 
+PLATFORM="$(uname -s | tr 'A-Z' 'a-z')"
+function is_windows() {
+  # On windows, the shell script is actually running in msys
+  if [[ "${PLATFORM}" =~ msys_nt* ]]; then
+    true
+  else
+    false
+  fi
+}
+
 function check_python {
   for x in $EXPECTED_PATHS; do
     if [ ! -e "$x" ]; then
       echo -e "\n\nERROR: Cannot find '${x}'.  Did you run configure?\n\n" 1>&2
       exit 1
     fi
-    if [ ! -L "${x}" ]; then
+    # Don't check symbolic link on Windows
+    if ! is_windows && [ ! -L "${x}" ]; then
       echo -e "\n\nERROR: '${x}' is not a symbolic link.  Internal error.\n\n" 1>&2
       exit 1
     fi