Merge branch 'jj1bdx-c_src-fix' of git://github.com/jj1bdx/erlang.mk
Add erl_interface for c_src compile and link

Using LDFLAGS, LDLIBS for link
Don't try compile c_src if not exist
Compile all *.c files in c_src and sub dir
Make env.mk delete on distclean

Because env.mk creates a considerable time, then delete it less often.