If anyone is interested - this is the Makefile I used to build Oblige 6 on Linux. It assumes that FLTK and libz are packaged with pkg-config. I kept the diff against Makefile.mingw as small as possible. I had to explicitly add -ljpeg as otherwise some symbols in libfltk_images failed to resolve. I was pleased I did not have to make any changes to the source.
BTW I am blown away by the improvement between Oblige versions 3 and 6. Version 6 really feels like Doom!