Add Boost dependency in configure

The configure.ac script requires Boost. The VLE 1.1 Win32 toolchain
splits Boost and VLE. So we add an m4 script to search boost on the
system.
rvle-1.1.0-beta1
win32: dynamically modify the path in onLoad

The windows path has to contain the path to vle install. To avoid modifying
the path for all applications, we set it in the R session before loading
the rvle.dll.