Only set SLIC3R_FHS_RESOURCES when SLIC3R_FHS is set
Only set SLIC3R_FHS_RESOURCES when SLIC3R_FHS is set

This makes it possible to run slic3r in a temporary direction
with

  make DESTDIR="run" install

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r