Rename *.c to *.cpp

So that we can use polly's data structure in ppcg. In the next step,
pet_stmt will be replaced with polly::ScopStmt. Other than renaming all
c files into cpp files, there are two main kinds of changes in the code:
 - We rename all the varialbes named 'private', because 'private' is
   used as keyword in C++.
 - We enforce pointer type casts for 'void *' to avoid build errors that
   exist in C++ but not in C.
Update makefile to include GPGPU codegen files into build

All the added GPGPU codegen files compiled successfully.
Do not check format of the GPGPU codegen sources.
Remove use of isl_printer parameter in "generate" function

The original generate function is divided into 2 parts:
 - the new generate function: store the generated ast_node host tree
   in gen and return void.

Two functions call or use "generate" as callback are commented out.
 - generate_wrap
 - generate_gpu

Add new declaration of generate_gpu to gpu.h.
Replace use of pet_stmt with polly::ScopStmt
Add an isl_id to MemoryAccess class.

This ref_id is used to represent an id of an access.
Replace pet_scop with polly::Scop

Some functions from pet/scop.c file is copied into ppcg.cpp and
modified with input of a polly::Scop pointer.
rename ppcg_scop_from_pet_scop to ppcg_scop_from_polly_scop
Add ppcg_scop_from_polly_scop to ppcg.h

Make the function an interface that can be called externally.
Remove static of some ppcg functions

Make the following functions as an external interface.
 - ppcg_scop_free
 - tag_access
Add ppcg_print_guarded to get guard ast_node of the host node.
Update extract_stmts function to avoid iterating all the pet_stmts

extract_stmts function is used to collect all the gpu_stmts. In ppcg,
every pet_stmt is iterated to look for gpu_stmt and especially those
access pet_expr (statements). In polly, we already have access info in
MemoryAccess. Thus, use MemoryAccess rather than pet_expr to
reimplemented extract_stmts function.
GPURuntime library update

 + Duplicate drvapi_error_string.h from cuda sdk to give much more
   readable error messages.
 + Add allocateMemoryForDevice function.
 - Fix the comments which are used to demonstrate usage of library.
Add IslPTXGenerator

IslPTXGenerator is used to generate llvm IR for both the host and
device.
Add struct gen_ext

This struct will be used to transmit all the needed outputs of the ppcg
generated intermediate representations - the optimized codes in isl
description.
 - guard: guard of the host code.
 - tree: host code ast_node.
 - prog: gpu_prog struct.
Update generate_gpu to use gen_ext

So we can use gen_ext to output the generated ast_node and gpu_prog.
Add two flags for GPGPU codegen(isl)

 - enable-polly-gpgpu-isl: enable polly GPGPU codegen.
 - polly-gpgpu-triple-isl: set the GPU triple string.
Add dump_ast_node debug option
Add print_ast_node_as_c_format function
Add an IslPTXGenerator member to IslNodeBuilder