graphics/goocanvas: the port had been improved (+)

• Define LICENSE (LGPL20)
• Do not manually set CPPFLAGS and LDFLAGS, USES+=localbase does the job nicely and is only needed in the NLS case anyways
• Convert to option helpers and INSTALL_TARGET=install-strip
• Fix documentation path (/usr/local/share/doc/goocanvas/goocanvas -> /usr/local/share/doc/goocanvas)