Rename PYTHONLIBS_FOUND to Python3_FOUND

src/CMakeLists.txt

-if(ADOLC_FOUND AND IPOPT_FOUND AND PYTHONLIBS_FOUND AND dune-uggrid_FOUND)
+if(ADOLC_FOUND AND IPOPT_FOUND AND Python3_FOUND AND dune-uggrid_FOUND)
   set(programs linear-elasticity)
 
   # When combining dune-common 2.7 and dune-fufem 2.8, no cmake test for the