xspec model library

At the request mainly of the ISIS folks I've managed to make a stripped-down HEAsoft 6.5 distribution which only includes the Xspec model library files to build only libXSFunctions, libXSUtil and libXS. I assume that the user has a cfitsio library already. Some minor hacks of the top level and Xspec configure files were necessary to make sure that hmake didn't try to build any heacore libraries. Also, moved some include files into Xspec/src/include from directories that otherwise aren't needed.