Fix for older app Makefiles

Makefiles based on our old examples typically pulled in object pattern
rules from Make.common but then used libmesh_CXX as the linker.  That
broke when we switched the obj-suffix to specify libtool rather than
raw object files.  This fixes it, still remains compatible with new
"fixed" app Makefiles which use libtool as the linker, and allows
newer Makefiles to easily choose one or the other method.