fastAssembly test script + -lrt for old libc in CMakeList

git-svn-id: https://geuz.org/svn/gmsh/trunk@18904 59c8cabf-10d6-4a28-b43f-3936b85956c4
Assembly and solving time: swave vs slow

git-svn-id: https://geuz.org/svn/gmsh/trunk@18905 59c8cabf-10d6-4a28-b43f-3936b85956c4