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.

Build/Configure.hs

@@ -1,4 +1,4 @@
-{- Checks system configuration and generates SysConfig.hs. -}
+{- Checks system configuration and generates SysConfig. -}
 
 {-# OPTIONS_GHC -fno-warn-tabs #-}