Don't include the headers for dynamically loading the VP library if NO_DYNAMIC_VP is defined

bundleloader.h, which is included if MACOS is defined, defines
inline functions that reference bundle loading system functions,
which requires linking to the core foundation framework.

Avoid requiring linking to extra libraries/frameworks if
NO_DYNAMIC_VP is defined.
Define MACOS when building for OS X

The library uses this define to enable OS X specific functionality
in general, even if none of those snippets actually have been
strictly required so far.
Merge pull request #185 from mstorsjo/macos-define

Define MACOS when building for OS X