boost::filesystem API change: no need to version filesystem to 3

this fixes the following error

>     boost::filesystem3 undeclared

It seems the people behind `boost` like to change the API every now and then..

I am using boost-libs 1.53.0 and `boost::filesystem3` is not accepted.
Instead from version 1.46 and above the default `boost::filesystem` seems to be version 3.

There also exists a flag to set the filesystem version `BOOST_FILESYSTEM_VERSION`
You may want to add this to the `CXXFLAGS` as `-DBOOST_FILESYSTEM_VERSION=3`.

[reference][r]

  [r]: http://www.boost.org/doc/libs/1_46_0/libs/filesystem/v3/doc/index.htm