[osg-ephemeris] optionally compiling with fltk2

[Terry Welsh]

I can compile with fltk2 instead of fltk1 without changing very much
in osgEphemeris, but it would be cool if osgEphemeris could do this
automagically.  I just don't know if makefiles can do it.

For the includes in Gui instead of looking in /usr/local/include/FL,
you would have to look in /usr/local/include/fltk/compat/FL.  This is
easy if you assume people installed fltk2 in the default /usr/local
paths, but it's harder if they installed somewhere else.  I don't know
how to replace just part of an include path.  In this case, you'd have
to optionally add "fltk/compat/FL" to other include paths.

For the link line in Gui fltk2 needs to have "-lfltk2 -lXft -lXi
-lXinerama" instead of just "-lfltk"   I don't know how to make this
optional, though.  Anyone know if this is possible to detect which lib
you have installed and use it.

Anyway, none of this is real important; I just thought it would be neat.
- Terry