Add P_LIB_INTERNAL_API and P_LIB_GLOBAL_API macros
Add P_LIB_INTERNAL_API and P_LIB_GLOBAL_API macros

These macros are based on the visibility attributes provided by various compilers.
To read more about symbols visibility and why to use it:
https://gcc.gnu.org/wiki/Visibility

saprykin/plibsys,saprykin/plib,saprykin/plib,saprykin/plibsys,saprykin/plibsys