system-cxx-std-lib: Add license and description

mk/system-cxx-std-lib-1.0.conf.in

@@ -3,7 +3,16 @@ version:              1.0
 visibility:           public
 id:                   system-cxx-std-lib-1.0
 key:                  system-cxx-std-lib-1.0
+license:              BSD-3-Clause
 synopsis:             A placeholder for the system's C++ standard library implementation.
+description:          Building against C++ libraries requires that the C++ standard
+                      library be included when linking. Typically when compiling a C++
+                      project this is done automatically by the C++ compiler. However,
+                      as GHC uses the C compiler for linking, users needing the C++
+                      standard library must declare this dependency explicitly.
+                      .
+                      This "virtual" package can be used to depend upon the host system's
+                      C++ standard library implementation in a platform agnostic manner.
 category:             System
 abi:                  00000000000000000000000000000000
 exposed:              True