Fix bug #461: Handles priorities are not working in some cases.

Place the comparison of the closest handle and the current handle outside the if block and modify the if block to use that comparison. It would prevent to do not take in consideration handles that are distant to the mouse position more than the closest handle found, but its distance is slightly bigger than the closest but significative small to be considered both at the same place.
Update Autoconf Boost detection scripts to the latest version
Tell build script to use synfig-core includes from the source dir. This should improve rebuild speed for synfig-studio.
Fixed problem with drag'n'drop files. Now synfig does import of dropped
files instead of creating a text layer with file URI.
Note: Non-ASCII characters in filenames or paths are not supported yet.
Import files by drag'n'drop now supports non-ASCII characters in
filenames and paths.
Fix #467: Localizations broken for Linux and OSX.
Linux package script: Enable required png target in ImageMagick
Little fix for "import by drag'n'drop" feature. Use Glibmm C++ functions
to parse file URI.
Changed warning message about unsupported URI
Typo
Fix OSX build script to work with MacPorts 2.2.0
Merge pull request #24 from morevnaproject/master

Fixes for build scripts from Konstantin Dmitriev and image drag-n-drop fix from Ivan Mahonin