this has only been build tested and is at the stage where it builds
with a couple of hacks.

- ${WRKSRC}/filters/{invert,linearblend}/Makefile

  -rpath needs an argument, and what gets put in the Makefile doesn't
  expand to anything. I don't know the qt/qmake/gmake magic needed to
  make it work. workaround is to hack the Makefiles and pass
  -rpath $(QTPREFIX)/lib

- hack the Makefiles and set MKDIR (mkdir) and CHK_DIR_EXISTS
  (test -d) to fix the install stage

- need to call libtool --install to install the resulting
  binaries/libraries (currently it installs the libtool wrappers, not
  the real binaries)

I suspect all of these can be fixed by playing with qt3/qmake somehow.
