[inaka/elvis#96] Elvis plugin, updated README.
Merge branch 'facorro.elvis.96.plugin.readme' of git://github.com/inaka/erlang.mk
Improve c_src handling

Don't don anything if no c_src dir.
Add erl_interface headers and lib for compile and link.
Make compile .c, .cc, .C, .cpp, .c++ to .o object using implicit rules.
Make link more make way (use $LDFLAGS $LOADLIBES $LDLIBS).