CPack NSIS fixes
Fixes in WIN32/MINGW32 compilation and packaging

* bundles a stripped version of x86_64-w64-mingw32-gfortran (4.8.3 sjlj)
  by installing directory ${CMAKE_CURRENT_SOURCE_DIR}/../stripped_gfortran
  verbatim
* installs windows batch scripts for elmerf90.bat and elmerld.bat
  that work with the stripped gfortran installation