Update pet.h with the up-to-date one from ppcg
Remove and comment unused header includes

pet.h has been included in ppcg.h thus no need to include it once again.
Change header include from <pet.h> to "pet.h"
Comment out main function
Update makefile to include GPGPU codegen files into build

All the added GPGPU codegen files compiled successfully.
Update format of the GPGPU codegen related files
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.
 - ppcg_print_guard: print the host end codes, e.g. the outermost
   non-parallel loops and if guards, not implemented yet.
 - gen_free: do the cleanup staff for gpu_gen struct.

Add declaration to gpu.h.

Two functions call or use "generate" as callback are commented out.
 - generate_wrap
 - generate_gpu
Make gpu_gen_free and ppcg_print_guard as static
Update format
Modify the return type value of generate function

Return 0 represents codegen successfully, return -1 otherwise.
Merge branch 'gpolly' of https://github.com/YabinHu/polly into gpolly