Bug 1260299 - Generate Visual Studio project files corresponding to current toolchain; r=chmanchester

If we're running VS2013, we generate VS2013 files. If we're running VS2015,
we generate VS2015 files. If we don't have a Visual Studio version
defined, refuse to generate project files (hopefully this doesn't
happen in the real world but I'm sure someone will complain if it does).

MozReview-Commit-ID: 5GdsbGmWPLB

--HG--
extra : rebase_source : 1a50c0fa5b7980ad25fb3ed3c1ec2c95f16996b6

python/mozbuild/mozbuild/backend/visualstudio.py

@@ -74,8 +74,10 @@ class VisualStudioBackend(CommonBackend):
         # These should eventually evolve into parameters.
         self._out_dir = os.path.join(self.environment.topobjdir, 'msvc')
         self._projsubdir = 'projects'
-        # But making this one a parameter requires testing first.
-        self._version = '2013'
+
+        self._version = self.environment.substs.get('MSVS_VERSION', None)
+        if not self._version:
+            raise Exception('MSVS_VERSION not defined; this should never happen')
 
         self._paths_to_sources = {}
         self._paths_to_includes = {}