This commit makes the default FLTK build setting use libdecor
as packaged in Linux when the build system contains packages
libdecor-0-dev and libdecor-0-plugin-1-gtk in version ≥ 0.2.0.
Otherwise, FLTK uses the bundled version of libdecor.
This includes situations where package libdecor-0-dev is
present in an earlier version.