Bug 1275297 - Write MinimumVisualStudioVersion to solution file; r=chmanchester

Visual Studio will write this variable to an ancient Visual Studio
version (2010 I believe) if we don't specify it. Explicitly write the
variable to the minimum Visual Studio version we support.

MozReview-Commit-ID: 8Y0im48OM2G

--HG--
extra : rebase_source : 7b1f4cfe778c6258dce068b2aec8b6acd8e85102

python/mozbuild/mozbuild/backend/visualstudio.py

@@ -261,6 +261,9 @@ class VisualStudioBackend(CommonBackend):
                  format_version)
         fh.write('# Visual Studio %s\r\n' % comment_version)
 
+        # Corresponds to VS2013.
+        fh.write('MinimumVisualStudioVersion = 12.0.31101.0\r\n')
+
         binaries_id = projects['target_binaries'][0]
 
         # Write out entries for each project.