Recognize MSVS 12 tools version for external projects too

According to https://msdn.microsoft.com/en-us/library/bb383796.aspx, this is
the native value used by MSVS 2013, but we didn't recognize it before,
presumably because we only tested with the external projects originally
created in a previous MSVS version and updated to VS 2013 later.

Do recognize 12, in addition to 4 (MSVS 2010 and 2012) and 14 (MSVS 2015) too
now.

src/bkl/plugins/external.py

@@ -172,6 +172,8 @@ def version(self):
         v = self.xml.get("ToolsVersion")
         if v == "14.0":
             return 14
+        elif v == "12.0":
+            return 12
         elif v != "4.0":
             raise Error("unrecognized version of Visual Studio project %s: ToolsVersion=\"%s\"" %(
                         self.projectfile, v))