Tiff and Geotiff CMake fixes.

Find routines were very old and naming was not conform to
find_package_handle_standard_args().
Cleanup.
Set correct include paths for Mac OS.

Removed FindLibTiff as this is provided by CMake.
Simplified FindLibGeoTiff.
Removed deprecated OGS_LIBS_DIR stuff.
Added option CMAKE_LIBRARY_SEARCH_PATH.

CMAKE_LIBRARY_SEARCH_PATH is to specify non-standard library
install locations. Can also be set as an environment variable.
More compact library search prefix path setting.
geotiff is now correctly found on Windows.
Fixing syntax error.
xtiff seems to be a separate library on Windows only.
Handle non-standard VTK install location on Windows.
Bumped VTK to 6.1.0

6.1.0 fixes a few compile errors on Mac and Linux.
Updated precompiled vtk 6 for Travis.
Fixed netcdf include path.
Removed empty line.
[CMake] Moved external lib urls to a separate file.
Added a CMake script for installing third-party libs (Windows).
Typos.
Fixed an Eigen CMake compile error on Windows.
renamed tpp-files to -impl.h
Bumped minimum Boost version to 1.48.0.

Because of compile errors in FileTools.cpp (tested 1.46, 1.47)