plugin/c_src: compile C++ source

Add compile .cc,.cpp,.C files as C++.
Compile each source file to .o.
Link all .o object to .so.
Delete .o on clean target.
plugin/c_src: remove C_SRC_OPTS

We can just LDFLAGS += and whatnot.
Merge branch 'C++_source' of git://github.com/nevar/erlang.mk
Update README for C/C++ compiler
Add missing variable to the README