remove Build/SysConfig.hs target

remove Build/SysConfig.hs target

The problem with that target was, if a target like git-annex that
depended on it failed for some reason, make would delete
Build/SysConfig.hs, since it knows it's an intermediate file. But, since
stack only builds that file once, that caused all subsequent make git-annex
builds to fail.

Also, this avoids a double stack build when building with stack. Since
stack has no configure stage, and the Build/SysConfig.hs target was
about running the configure stage, the only way to only build once is to
combine the targets like this.

This should work better on the autobuilders that build with stack.

This commit was sponsored by NSF-funded DataLad project