Fix build with BUILD_SHARED_LIBS=ON

[MiguelCompany]

This should fix #131 by deprecating virtual_memory_page_size and adding function get_virtual_memory_page_size().

[foonathan]

Hm, this is a breaking change. Maybe it's better to call the function get_virtual_memory_page_size() and have a (deprecated) static const std::size_t virtual_memory_page_size = get_virtual_memory_page_size()? That being said, it probably doesn't matter. I leave the decision up to you.

[MiguelCompany]

I do prefer non-breaking changes, so I just added the getter function that returns the constant.

[foonathan]

Thank you!