cpp/python: export empty and universe constructors for sets

These functions are documented and commonly used when constructing isl
sets. Export them for basic sets, sets, and union sets:

            isl_basic_set_empty
            isl_basic_set_universe

            isl_set_empty
            isl_set_universe

            isl_union_set_empty

As the arguments of these constructors do not always uniquely identify
the constructed object, named constructors are introduced according to
the isl interface guidelines.

Signed-off-by: Tobias Grosser <tobias@grosser.es>
Reviewed-by: Oleksandr Zinenko <oleksandr.zinenko@inria.fr>

include/isl/set.h

@@ -91,7 +91,9 @@ int isl_basic_set_is_rational(__isl_keep isl_basic_set *bset);
 
 __isl_null isl_basic_set *isl_basic_set_free(__isl_take isl_basic_set *bset);
 __isl_give isl_basic_set *isl_basic_set_copy(__isl_keep isl_basic_set *bset);
+__isl_export
 __isl_give isl_basic_set *isl_basic_set_empty(__isl_take isl_space *space);
+__isl_export
 __isl_give isl_basic_set *isl_basic_set_universe(__isl_take isl_space *space);
 __isl_give isl_basic_set *isl_basic_set_nat_universe(__isl_take isl_space *dim);
 __isl_give isl_basic_set *isl_basic_set_positive_orthant(
@@ -236,7 +238,9 @@ isl_bool isl_basic_set_is_subset(__isl_keep isl_basic_set *bset1,
 isl_bool isl_basic_set_plain_is_equal(__isl_keep isl_basic_set *bset1,
 	__isl_keep isl_basic_set *bset2);
 
+__isl_export
 __isl_give isl_set *isl_set_empty(__isl_take isl_space *space);
+__isl_export
 __isl_give isl_set *isl_set_universe(__isl_take isl_space *space);
 __isl_give isl_set *isl_set_nat_universe(__isl_take isl_space *dim);
 __isl_give isl_set *isl_set_copy(__isl_keep isl_set *set);

include/isl/union_set.h

@@ -16,6 +16,7 @@ __isl_give isl_union_set *isl_union_set_from_basic_set(
 	__isl_take isl_basic_set *bset);
 __isl_constructor
 __isl_give isl_union_set *isl_union_set_from_set(__isl_take isl_set *set);
+__isl_export
 __isl_give isl_union_set *isl_union_set_empty(__isl_take isl_space *space);
 __isl_give isl_union_set *isl_union_set_copy(__isl_keep isl_union_set *uset);
 __isl_null isl_union_set *isl_union_set_free(__isl_take isl_union_set *uset);