Restore Makefile.

Change-Id: I63f2665a7ce9bc81729dd0978974b6f0f269bdd0
Allow specifying host back-end for pencil_alloc.

Change-Id: I686b0f5696c5bfac603da56b1b77d2da00def418
Support building OpenCL program from string.

Change-Id: Ib3cdd4cfd781fcc67b6290d2d16a2f08500436fa