[CPML] Added CPML_GNUC_CONST macro
[CPML] Added CPML_GNUC_CONST macro

Implemented a macro to optimize gcc with const return values (the
same as G_GNUC_CONST in glib). This is needed as the get_npoints
implementation (the number of points needed to define a primitive)
is a constant value.

ntd/adg,ntd/adg