Add "default" target in Makefile, which builds nothing.

The "install" target in Makefile contains some scripts to copy
libs and configs to dest dir.

Actually, `make install` is enough. But,  the ebuild of protage
always call `src_compile` and `src_install` during emerging.
So, put "default" session before "install" session in Makefile,
then `src_compile` will build "default" session, which compiles nothing,
and `src_install` will "install" the libs and configs