Only use git describe as version string if .git dir exists

We don't want to pick up the git version string on distros
which have build scripts in git.

wscript

@@ -35,6 +35,7 @@ APPNAME = 'midori'
 VERSION = str (major) + '.' + str (minor) + '.' + str (micro)
 
 try:
+    if os.path.isdir ('.git'):
         git = Utils.cmd_output (['git', 'describe'], silent=True)
         if git:
             VERSION = git.strip ()