Add builtin constants ‘langVersion’ and ‘nixVersion’

The integer constant ‘langVersion’ denotes the current language
version.  It gets increased every time a language feature is
added/changed/removed.  It's currently 1.

The string constant ‘nixVersion’ contains the current Nix version,
e.g. "1.2pre2980_9de6bc5".

src/libstore/globals.cc

@@ -197,4 +197,7 @@ Settings::SettingsMap Settings::getOverrides()
 }
 
 
+const string nixVersion = NIX_VERSION;
+
+
 }