add rule for clean (#4364)
add rule for clean (#4364)

* add rule for clean

* Update clean rule

Seems like lib/ directory is not made by the makefile
So don't delete directory, just the contents of it.

Laurawly/tvm-1,sxjscience/tvm,dmlc/tvm,sxjscience/tvm,sxjscience/tvm,Laurawly/tvm-1,dmlc/tvm,sxjscience/tvm,tqchen/tvm,tqchen/tvm,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,dmlc/tvm,sxjscience/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,Laurawly/tvm-1,tqchen/tvm,sxjscience/tvm,tqchen/tvm,dmlc/tvm,tqchen/tvm,dmlc/tvm,sxjscience/tvm,tqchen/tvm,sxjscience/tvm,tqchen/tvm,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,tqchen/tvm