add NO_AUTO_CLEAN to needs_clean

Since the default Makefile in R doesn't handle dependencies of header
files, devtools run a clean compile when a header or Makevars is newer
than the dll.

Based on (http://bruno.defraine.net/techtips/makefile-auto-dependencies-with-gcc/),
I use the following Makevars to handle dependencies in development
environment. It may improve compiling speed significant when a header
file is modified.

```make
 # DEVTOOLS_NO_AUTO_CLEAN
PKG_CXXFLAGS=-MMD -MP

all: $(SHLIB)

SRC=$(wildcard *.cpp)
DEP=$(SRC:.cpp=.d)
-include $(DEP)
```

With this commit, needs_clean will not trigger cleaning and let the
Makevars handle dependencies of header files, when it see a commented
line `# DEVTOOLS_NO_AUTO_CLEAN` in Makevars.