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.

Didn't test:
* flags for FreeBSD and Darwin
* not sure that add $(C_SRC_OPTS) to C and C++ compile options

Didn't add compile c++ as it look like default extension for Windows.
I know it's silly, but SOURCE and OBJECT not being plural doesn't seem right.