Add NO_DYNAMIC_VP when building the encoder lib for the iOS simulator

This was previously only added to builds for actual devices.
Enable threading when building for iOS using the project files

This was already enabled when building the library for iOS using
the makefiles.
Merge pull request #457 from mstorsjo/ios-enc-defines

Add missing defines when building the encoder for iOS using the xcode project files
Merge remote-tracking branch 'upstream/master' into travis.osx
Merge branch 'travis.osx' of https://github.com/ethanhugg/openh264 into travis.osx