make bindist: Simplify FBMANIFEST var
make bindist: Add FBPACKSUFFIX var for extending the package name

This is useful when making multiple packages for the same version & target,
such that we need an additional suffix to distinguish them, for example:
	FreeBASIC-x.xx.x-win32 ("normal" win32 build)
	FreeBASIC-x.xx.x-win32-mingworg (MinGW.org build)