Build system option for static PDAL build

Add an option (off by default) to build pdal as a static library.  I don't know how useful this really is because it requires plugins to be builtin, but it will get me unstuck for now.

Note that I've **removed** -fPIC from the generic common options, because that's counter productive for a static build.  However, it's required when linking the static embedded boost into a dynamic pdal lib, so I made a change to put it in there.  @hobu I hope that works for you - I see that you added -fPIC in 9855c9a0c ?