compilation directives (cf. __APPLE__), were to rename the type
ColorTable to GmshColorTable and the function Palette to Palette1,
in order to avoid conflicts with Apple's QuickDraw.
Note that HFS is case-insensitive, which requires to have a fltk lib
name otherwise than libfltk.a (in the Mac version, the GL part is separate
anyway, so I've been using libfltk_base.a and libfltk_gl.a).

Interface polish (buttons should not be resized when the size of the window is changed);
gl2ps update (not finished);