Fix compilation on Ubuntu precise.

This distro ships glibc-2.15 where clock_gettime() is part of the
realtime library librt. So all executables which call clock_gettime()
must be linked with -lrt. The configure script detects this and adds
-lrt to LDFLAGS. However, -lrt must come *after* the objects to be
linked together, which is currently not the case.

Fix this by moving $(LDFLAGS) to the end of the recipe for the targets
of all executables.