Find the native FLTK2 includes and library

The following settings are defined

FLTK2_FLUID_EXECUTABLE, where to find the Fluid tool
FLTK2_WRAP_UI, This enables the FLTK2_WRAP_UI command
FLTK2_INCLUDE_DIR, where to find include files
FLTK2_LIBRARIES, list of fltk2 libraries
FLTK2_FOUND, Don't use FLTK2 if false.

The following settings should not be used in general.

FLTK2_BASE_LIBRARY   = the full path to fltk2.lib
FLTK2_GL_LIBRARY     = the full path to fltk2_gl.lib
FLTK2_IMAGES_LIBRARY = the full path to fltk2_images.lib