Fix compiler warnings generated by Clang 3.4

-- Fix compiler warning (-Wreorder):
   Change the order of the constructor initialization list.

-- Fix compiler warning (-Wunused):
   Comment out unused variable.

-- Change "class" to struct to match previous declaration.

-- Remove deprecated register keyword.
Add ifdef's around QDP_debug().

-- Add ifdef GPU_DEBUG macors around QDP_debug. The macro
   is used in the definition of QDP_debug in qdp_utils.cc. If
   we compile without this flag then the QDP_debug function
   ends up with and empty body. This causes a runtime crash
   with LLVM that complains of illegal instruction.
Change the signature of the find_attr function.

-- Change the return type to void from bool.
   Otherwise LLVM suffers a runtime crash.
Fixes a compiler error in qdp_subset.cc