Allow VegaFEM to be used by external packages.

VegaFEM now installs libraries plus CMake config
and target files so that it can be used by other packages.

PS. The duplicate headers in include and src/libXXX are evil!

PPS. The Vega getopts library has been renamed libvega-getopts
     because of issues with the preprocessor and potential
     install-name conflicts/confusion with the real getopt.