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.

Git/BuildVersion.hs

@@ -8,14 +8,14 @@
 module Git.BuildVersion where
 
 import Git.Version
-import qualified Build.SysConfig
+import qualified BuildInfo
 
 {- Using the version it was configured for avoids running git to check its
  - version, at the cost that upgrading git won't be noticed.
  - This is only acceptable because it's rare that git's version influences
  - code's behavior. -}
 buildVersion :: GitVersion
-buildVersion = normalize Build.SysConfig.gitversion
+buildVersion = normalize BuildInfo.gitversion
 
 older :: String -> Bool
 older n = buildVersion < normalize n