Handle projects.h not being available

Starting with proj4 4.8.0 the projects.h header is no longer intended to be installed - the NEWS file says:

* projects.h no longer installed as a public include file.  Please try to only use proj_api.h. 

Ubuntu at least has now stopped including it in their package, though Fedora still does.

This PR allows proj4rb to detect when the header is missing - unfortunately certain methods which rely on access to the library internals then have to be disabled.

I'm not quite sure if this is the right place to send this change though, as rubygems.org has a 0.4.3 release which doesn't seem to be in your repo, but I haven't managed to work out where it did come from.