Include pencil_lib.h from pencil_compat.h

Suggested by Albert.  Having only one header file for the PENCIL
compatibility mode is better than having two files (in case a user wants
only to use PENCIL in compatibility mode, i.e. to compile his code with
a standard C compiler).  This patch includes the automatically
generated pencil_lib.h file in pencil_compat.h so that the user
does not need to think about including both.