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.
Merge branch 'c_src_hanling' of git://github.com/nevar/erlang.mk
Enabled the c_src plugin by default