Avoid all calls to libdl (or equivalent) if NO_DYNAMIC_VP is defined

The same ifdefs are already used in the corresponding constructors,
so this just avoids a runtime/link time library dependency which
is practically unused in this build setup.
Merge pull request #45 from mstorsjo/no-libdl

Avoid all calls to libdl (or equivalent) if NO_DYNAMIC_VP is defined