Improve git lookup by using which() to search PATH if git is not installed in emsdk.

emsdk

@@ -297,6 +297,32 @@ def run_get_output(cmd, cwd=None):
   (stdout, stderr) = process.communicate()
   return (process.returncode, stdout, stderr)
 
+# Finds the given executable 'program' in PATH. Operates like the Unix tool 'which'.
+def which(program):
+  def is_exe(fpath):
+    return os.path.isfile(fpath) and os.access(fpath, os.X_OK)
+
+  fpath, fname = os.path.split(program)
+  if fpath:
+    if is_exe(program):
+      return program
+  else:
+    for path in os.environ["PATH"].split(os.pathsep):
+      path = path.strip('"')
+      exe_file = os.path.join(path, program)
+      if is_exe(exe_file):
+        return exe_file
+
+      if WINDOWS and not '.' in fname:
+        if is_exe(exe_file + '.exe'):
+          return exe_file + '.exe'
+        if is_exe(exe_file + '.cmd'):
+          return exe_file + '.cmd'
+        if is_exe(exe_file + '.bat'):
+          return exe_file + '.bat'
+
+  return None
+
 warnonce_git_not_found = False
 
 def GIT():
@@ -308,7 +334,7 @@ def GIT():
       return git
   except:
     pass
-  git = 'git'
+  git = which('git')
   try:
     (ret, stdout, stderr) = run_get_output([git, '--version'])
     if ret == 0: