Export isl_val arithmetic and comparison methods

Export isl_val arithmetic and comparison methods

These functions have been tested and work well with the python bindings.
Three sets of functions are currently not exported:

	1) The integer versions of certain functions

	Isl currently provides two functions:

		isl_val_sub(isl_val *v1, isl_val *v2);
		isl_val_sub_ui(isl_val *v1, unsigned long);

	We only export the first function. The second function is not
        necessary as isl automatically upgrades integers to an isl_val

	2) isl_val_2exp

	Method names are not allowed to start with numbers, which
	prevents us to export a method '2exp'. We may export this
	function in a future change that explicitly addresses this
        problem.

	3) isl_val_gcdext

	This function returns two values by taking two reference
	arguments.

Signed-off-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>