fold Build/SysConfig.hs into BuildInfo via include

This avoids warnings from stack about the module not being listed in the
cabal file. So, the generated file is also renamed to Build/SysConfig.

Note that the setup program seems to be cached despite these changes; I
had to cabal clean to get cabal to update it so that Build/SysConfig was
written.

This commit was sponsored by Jochen Bartl on Patreon.

BuildInfo.hs

@@ -1,4 +1,4 @@
-{- git-annex build info reporting
+{- git-annex build info
  -
  - Copyright 2013-2017 Joey Hess <id@joeyh.name>
  -
@@ -13,6 +13,8 @@ import Data.List
 import Data.Ord
 import qualified Data.CaseInsensitive as CI
 
+#include "Build/SysConfig"
+
 buildFlags :: [String]
 buildFlags = filter (not . null)
 	[ ""