grid-config: Expose CXXLD (for GPU build) and update help

grid-config.in

@@ -19,6 +19,8 @@ Known values for OPTION are:
   --help       display this help and exit
   --version    output version information
   --git        print git revision
+  --cxx        print c++ compiler and compiler flags
+  --cxxld      print c++ linker   and linker   flags
 
 EOF
   
@@ -65,6 +67,10 @@ while test $# -gt 0; do
       echo @GRID_CXX@
     ;;
     
+    --cxxld)
+      echo @CXXLD@
+    ;;
+    
     --ldflags)
       echo @GRID_LDFLAGS@
     ;;